Replace hashicorp/go-multierror by own implementation

The own implementation is much easier as it only has to serve our one
use case.

prometheus/promhttp/http.go

@@ -82,18 +82,18 @@ func HandlerFor(reg prometheus.Gatherer, opts HandlerOpts) http.Handler {
 		mfs, err := reg.Gather()
 		if err != nil {
 			if opts.ErrorLog != nil {
-				opts.ErrorLog.Println("error collecting metrics:", err)
+				opts.ErrorLog.Println("error gathering metrics:", err)
 			}
 			switch opts.ErrorHandling {
 			case PanicOnError:
 				panic(err)
 			case ContinueOnError:
 				if len(mfs) == 0 {
-					http.Error(w, "No metrics collected, last error:\n\n"+err.Error(), http.StatusInternalServerError)
+					http.Error(w, "No metrics gathered, last error:\n\n"+err.Error(), http.StatusInternalServerError)
 					return
 				}
 			case HTTPErrorOnError:
-				http.Error(w, "An error has occurred during metrics collection:\n\n"+err.Error(), http.StatusInternalServerError)
+				http.Error(w, "An error has occurred during metrics gathering:\n\n"+err.Error(), http.StatusInternalServerError)
 				return
 			}
 		}

prometheus/promhttp/http_test.go

@@ -88,14 +88,12 @@ func TestHandlerErrorHandling(t *testing.T) {
 		ErrorLog:      logger,
 		ErrorHandling: PanicOnError,
 	})
-	wantMsg := `error collecting metrics: 1 error(s) occurred:
-
+	wantMsg := `error gathering metrics: 1 error(s) occurred:
 * error collecting metric Desc{fqName: "invalid_metric", help: "not helpful", constLabels: {}, variableLabels: []}: collect error
 `
-	wantErrorBody := `An error has occurred during metrics collection:
+	wantErrorBody := `An error has occurred during metrics gathering:
 
 1 error(s) occurred:
-
 * error collecting metric Desc{fqName: "invalid_metric", help: "not helpful", constLabels: {}, variableLabels: []}: collect error
 `
 	wantOKBody := `# HELP name docstring

prometheus/registry.go

@@ -14,6 +14,7 @@
 package prometheus
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 	"os"
@@ -21,7 +22,6 @@ import (
 	"sync"
 
 	"github.com/golang/protobuf/proto"
-	"github.com/hashicorp/go-multierror"
 
 	dto "github.com/prometheus/client_model/go"
 )
@@ -220,6 +220,22 @@ func (err AlreadyRegisteredError) Error() string {
 	return "duplicate metrics collector registration attempted"
 }
 
+// MultiError is a slice of errors implementing the error interface. It is used
+// by a Gatherer to report multiple errors during MetricFamily gathering.
+type MultiError []error
+
+func (errs MultiError) Error() string {
+	if len(errs) == 0 {
+		return ""
+	}
+	buf := &bytes.Buffer{}
+	fmt.Fprintf(buf, "%d error(s) occurred:", len(errs))
+	for _, err := range errs {
+		fmt.Fprintf(buf, "\n* %s", err)
+	}
+	return buf.String()
+}
+
 // Registry registers Prometheus collectors, collects their metrics, and gathers
 // them into MetricFamilies for exposition. It implements Registerer and
 // Gatherer. The zero value is not usable. Create instances with NewRegistry or
@@ -366,7 +382,7 @@ func (r *Registry) Gather() ([]*dto.MetricFamily, error) {
 		metricChan        = make(chan Metric, capMetricChan)
 		metricHashes      = map[uint64]struct{}{}
 		wg                sync.WaitGroup
-		errs              error               // The collected errors to return in the end.
+		errs              MultiError          // The collected errors to return in the end.
 		registeredDescIDs map[uint64]struct{} // Only used for pedantic checks
 	)
 
@@ -419,7 +435,7 @@ func (r *Registry) Gather() ([]*dto.MetricFamily, error) {
 		}
 		dtoMetric := &dto.Metric{}
 		if err := metric.Write(dtoMetric); err != nil {
-			errs = multierror.Append(errs, fmt.Errorf(
+			errs = append(errs, fmt.Errorf(
 				"error collecting metric %v: %s", desc, err,
 			))
 			continue
@@ -438,13 +454,13 @@ func (r *Registry) Gather() ([]*dto.MetricFamily, error) {
 		case dtoMetric.Histogram != nil:
 			metricFamily.Type = dto.MetricType_HISTOGRAM.Enum()
 		default:
-			errs = multierror.Append(errs, fmt.Errorf(
+			errs = append(errs, fmt.Errorf(
 				"empty metric collected: %s", dtoMetric,
 			))
 			continue
 		}
 		if err := r.checkConsistency(metricFamily, dtoMetric, desc, metricHashes, registeredDescIDs); err != nil {
-			errs = multierror.Append(errs, err)
+			errs = append(errs, err)
 			continue
 		}
 		metricFamily.Metric = append(metricFamily.Metric, dtoMetric)
@@ -464,7 +480,7 @@ func (r *Registry) Gather() ([]*dto.MetricFamily, error) {
 			}
 			for _, m := range mf.Metric {
 				if err := r.checkConsistency(existingMF, m, nil, metricHashes, nil); err != nil {
-					errs = multierror.Append(errs, err)
+					errs = append(errs, err)
 					continue
 				}
 				existingMF.Metric = append(existingMF.Metric, m)
@@ -493,6 +509,11 @@ func (r *Registry) Gather() ([]*dto.MetricFamily, error) {
 	for _, name := range names {
 		result = append(result, metricFamiliesByName[name])
 	}
+	// We cannot just `return result, errs`. Even if errs == nil, it will
+	// not be seen as nil through the error interface.
+	if len(errs) == 0 {
+		return result, nil
+	}
 	return result, errs
 }