Add a package promauto that provides auto-registering metrics

Finally, I found an easy solution to provide the "evil"
auto-registration without getting death threats from the wardens of Go
purity. The reasoning can be found in the package's doc comment.

prometheus/doc.go

@@ -14,7 +14,9 @@
 // Package prometheus is the core instrumentation package. It provides metrics
 // primitives to instrument code for monitoring. It also offers a registry for
 // metrics. Sub-packages allow to expose the registered metrics via HTTP
-// (package promhttp) or push them to a Pushgateway (package push).
+// (package promhttp) or push them to a Pushgateway (package push). There is
+// also a sub-package promauto, which provides metrics constructors with
+// automatic registration.
 //
 // All exported functions and methods are safe to be used concurrently unless
 // specified otherwise.
@@ -72,7 +74,10 @@
 // The number of exported identifiers in this package might appear a bit
 // overwhelming. However, in addition to the basic plumbing shown in the example
 // above, you only need to understand the different metric types and their
-// vector versions for basic usage.
+// vector versions for basic usage. Furthermore, if you are not concerned with
+// fine-grained control of when and how to register metrics with the registry,
+// have a look at the promauto package, which will effectively allow you to
+// ignore registration altogether in simple cases.
 //
 // Above, you have already touched the Counter and the Gauge. There are two more
 // advanced metric types: the Summary and Histogram. A more thorough description

prometheus/promauto/auto.go

@@ -0,0 +1,204 @@
+// Copyright 2018 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package promauto provides constructors for the usual Prometheus metrics that
+// return them already registered with the global registry
+// (prometheus.DefaultRegisterer). This allows very compact code, avoiding any
+// references to the registry altogether, but all the constructors in this
+// package will panic if the registration fails.
+//
+// The following example is a complete program to create a histogram of normally
+// distributed random numbers from the math/rand package:
+//
+//      package main
+//
+//      import (
+//              "math/rand"
+//              "net/http"
+//
+//              "github.com/prometheus/client_golang/prometheus"
+//              "github.com/prometheus/client_golang/prometheus/promauto"
+//              "github.com/prometheus/client_golang/prometheus/promhttp"
+//      )
+//
+//      var histogram = promauto.NewHistogram(prometheus.HistogramOpts{
+//              Name:    "random_numbers",
+//              Help:    "A histogram of normally distributed random numbers.",
+//              Buckets: prometheus.LinearBuckets(-3, .1, 61),
+//      })
+//
+//      func Random() {
+//              for {
+//                      histogram.Observe(rand.NormFloat64())
+//              }
+//      }
+//
+//      func main() {
+//              go Random()
+//              http.Handle("/metrics", promhttp.Handler())
+//              http.ListenAndServe(":1971", nil)
+//      }
+//
+// Prometheus's version of a minimal hello-world program:
+//
+//      package main
+//
+//      import (
+//      	"fmt"
+//      	"net/http"
+//
+//      	"github.com/prometheus/client_golang/prometheus"
+//      	"github.com/prometheus/client_golang/prometheus/promauto"
+//      	"github.com/prometheus/client_golang/prometheus/promhttp"
+//      )
+//
+//      func main() {
+//      	http.Handle("/", promhttp.InstrumentHandlerCounter(
+//      		promauto.NewCounterVec(
+//      			prometheus.CounterOpts{
+//      				Name: "hello_requests_total",
+//      				Help: "Total number of hello-world requests by HTTP code.",
+//      			},
+//      			[]string{"code"},
+//      		),
+//      		http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+//      			fmt.Fprint(w, "Hello, world!")
+//      		}),
+//      	))
+//      	http.Handle("/metrics", promhttp.Handler())
+//      	http.ListenAndServe(":1971", nil)
+//      }
+//
+// This appears very handy. So why are these constructors locked away in a
+// separate package? There are two caveats:
+//
+// First, in more complex programs, global state is often quite problematic.
+// That's the reason why the metrics constructors in the prometheus package do
+// not interact with the global prometheus.DefaultRegisterer on their own. You
+// are free to use the Register or MustRegister functions to register them with
+// the global prometheus.DefaultRegisterer, but you could as well choose a local
+// Registerer (usually created with prometheus.NewRegistry, but there are other
+// scenarios, e.g. testing).
+//
+// The second issue is that registration may fail, e.g. if a metric inconsistent
+// with the newly to be registered one is already registered. But how to signal
+// and handle a panic in the automatic registration with the default registry?
+// The only way is panicking. While panicking on invalid input provided by the
+// programmer is certainly fine, things are a bit more subtle in this case: You
+// might just add another package to the program, and that package (in its init
+// function) happens to register a metric with the same name as your code. Now,
+// all of a sudden, either your code or the code of the newly imported package
+// panics, depending on initialization order, without any opportunity to handle
+// the case gracefully. Even worse is a scenario where registration happens
+// later during the runtime (e.g. upon loading some kind of plugin), where the
+// panic could be triggered long after the code has been deployed to
+// production. A possibility to panic should be explicitly called out by the
+// Must… idiom, cf. prometheus.MustRegister. But adding a separate set of
+// constructors in the prometheus package called MustRegisterNewCounterVec or
+// similar would be quite unwieldy. Adding an extra MustRegister method to each
+// metric, returning the registered metric, would result in nice code for those
+// using the method, but would pollute every single metric interface for
+// everybody avoiding the global registry.
+//
+// To address both issues, the problematic auto-registering and possibly
+// panicking constructors are all in this package with a clear warning
+// ahead. And whoever cares about avoiding global state and possibly panicking
+// function calls can simply ignore the existence of the promauto package
+// altogether.
+//
+// A final note: There is a similar case in the net/http package of the standard
+// library. It has DefaultServeMux as a global instance of ServeMux, and the
+// Handle function acts on it, panicking if a handler for the same pattern has
+// already been registered. However, one might argue that the whole HTTP routing
+// is usually set up closely together in the same package or file, while
+// Prometheus metrics tend to be spread widely over the codebase, increasing the
+// chance of surprising registration failures. Furthermore, the use of global
+// state in net/http has been criticized widely, and some avoid it altogether.
+package promauto
+
+import "github.com/prometheus/client_golang/prometheus"
+
+// NewCounter works like the function of the same name in the prometheus package
+// but it automatically registers the Counter with the
+// prometheus.DefaultRegisterer. If the registration fails, NewCounter panics.
+func NewCounter(opts prometheus.CounterOpts) prometheus.Counter {
+	c := prometheus.NewCounter(opts)
+	prometheus.MustRegister(c)
+	return c
+}
+
+// NewCounterVec works like the function of the same name in the prometheus
+// package but it automatically registers the CounterVec with the
+// prometheus.DefaultRegisterer. If the registration fails, NewCounterVec
+// panics.
+func NewCounterVec(opts prometheus.CounterOpts, labelNames []string) *prometheus.CounterVec {
+	c := prometheus.NewCounterVec(opts, labelNames)
+	prometheus.MustRegister(c)
+	return c
+}
+
+// NewGauge works like the function of the same name in the prometheus package
+// but it automatically registers the Gauge with the
+// prometheus.DefaultRegisterer. If the registration fails, NewGauge panics.
+func NewGauge(opts prometheus.GaugeOpts) prometheus.Gauge {
+	g := prometheus.NewGauge(opts)
+	prometheus.MustRegister(g)
+	return g
+}
+
+// NewGaugeVec works like the function of the same name in the prometheus
+// package but it automatically registers the GaugeVec with the
+// prometheus.DefaultRegisterer. If the registration fails, NewGaugeVec panics.
+func NewGaugeVec(opts prometheus.GaugeOpts, labelNames []string) *prometheus.GaugeVec {
+	g := prometheus.NewGaugeVec(opts, labelNames)
+	prometheus.MustRegister(g)
+	return g
+}
+
+// NewSummary works like the function of the same name in the prometheus package
+// but it automatically registers the Summary with the
+// prometheus.DefaultRegisterer. If the registration fails, NewSummary panics.
+func NewSummary(opts prometheus.SummaryOpts) prometheus.Summary {
+	s := prometheus.NewSummary(opts)
+	prometheus.MustRegister(s)
+	return s
+}
+
+// NewSummaryVec works like the function of the same name in the prometheus
+// package but it automatically registers the SummaryVec with the
+// prometheus.DefaultRegisterer. If the registration fails, NewSummaryVec
+// panics.
+func NewSummaryVec(opts prometheus.SummaryOpts, labelNames []string) *prometheus.SummaryVec {
+	s := prometheus.NewSummaryVec(opts, labelNames)
+	prometheus.MustRegister(s)
+	return s
+}
+
+// NewHistogram works like the function of the same name in the prometheus
+// package but it automatically registers the Histogram with the
+// prometheus.DefaultRegisterer. If the registration fails, NewHistogram panics.
+func NewHistogram(opts prometheus.HistogramOpts) prometheus.Histogram {
+	h := prometheus.NewHistogram(opts)
+	prometheus.MustRegister(h)
+	return h
+}
+
+// NewHistogramVec works like the function of the same name in the prometheus
+// package but it automatically registers the HistogramVec with the
+// prometheus.DefaultRegisterer. If the registration fails, NewHistogramVec
+// panics.
+func NewHistogramVec(opts prometheus.HistogramOpts, labelNames []string) *prometheus.HistogramVec {
+	h := prometheus.NewHistogramVec(opts, labelNames)
+	prometheus.MustRegister(h)
+	return h
+}