Provide Discriminator For Protocol Buf. Streams

This commit introduces an incomplete processor that decodes varint
record length-delimited streams of io.prometheus.client.MetricFamily
Protocol Buffer messages.  The Go client presently does not support
generation of said messages, but this will unblock a number of Java
changes.

extraction/discriminator.go

@@ -31,10 +31,17 @@ func ProcessorForRequestHeader(header http.Header) (Processor, error) {
 	if err != nil {
 		return nil, fmt.Errorf("Invalid Content-Type header %q: %s", header.Get("Content-Type"), err)
 	}
-	if mediatype != "application/json" {
-		return nil, fmt.Errorf("Unsupported media type %q, expected %q", mediatype, "application/json")
+	switch mediatype {
+	case "application/vnd.google.protobuf":
+		if params["proto"] != "io.prometheus.client.MetricFamily" {
+			return nil, fmt.Errorf("Unrecognized Protocol Message %s", params["proto"])
 		}
+		if params["encoding"] != "varint record length-delimited" {
+			return nil, fmt.Errorf("Unsupported Encoding %s", params["encoding"])
+		}
+		return MetricFamilyProcessor, nil
 
+	case "application/json":
 		var prometheusApiVersion string
 
 		if params["schema"] == "prometheus/telemetry" && params["version"] != "" {
@@ -51,4 +58,7 @@ func ProcessorForRequestHeader(header http.Header) (Processor, error) {
 		default:
 			return nil, fmt.Errorf("Unrecognized API version %s", prometheusApiVersion)
 		}
+	default:
+		return nil, fmt.Errorf("Unsupported media type %q, expected %q", mediatype, "application/json")
+	}
 }

extraction/discriminator_test.go

@@ -56,6 +56,21 @@ func testDiscriminatorHttpHeader(t test.Tester) {
 			output: Processor002,
 			err:    nil,
 		},
+		{
+			input:  map[string]string{"Content-Type": `application/vnd.google.protobuf; proto="io.prometheus.client.MetricFamily"; encoding="varint record length-delimited"`},
+			output: MetricFamilyProcessor,
+			err:    nil,
+		},
+		{
+			input:  map[string]string{"Content-Type": `application/vnd.google.protobuf; proto="illegal"; encoding="varint record length-delimited"`},
+			output: nil,
+			err:    fmt.Errorf("Unrecognized Protocol Message illegal"),
+		},
+		{
+			input:  map[string]string{"Content-Type": `application/vnd.google.protobuf; proto="io.prometheus.client.MetricFamily"; encoding="illegal"`},
+			output: nil,
+			err:    fmt.Errorf("Unsupported Encoding illegal"),
+		},
 	}
 
 	for i, scenario := range scenarios {

extraction/metricfamilyprocessor.go

@@ -0,0 +1,31 @@
+// Copyright 2013 Prometheus Team
+// 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 extraction
+
+import (
+	"io"
+)
+
+type metricFamilyProcessor struct{}
+
+// MetricFamilyProcessor decodes varint encoded record length-delimited streams
+// of io.prometheus.client.MetricFamily.
+//
+// See http://godoc.org/github.com/matttproud/golang_protobuf_extensions/ext for
+// more details.
+var MetricFamilyProcessor = new(metricFamilyProcessor)
+
+func (m *metricFamilyProcessor) ProcessSingle(io.Reader, chan<- *Result, *ProcessOptions) error {
+	panic("not implemented")
+}

extraction/processor0_0_2.go

@@ -23,7 +23,7 @@ import (
 
 // Processor002 is responsible for decoding payloads from protocol version
 // 0.0.2.
-var Processor002 = &processor002{}
+var Processor002 = new(processor002)
 
 type histogram002 struct {
 	Labels map[string]string            `json:"labels"`