Add benchmark for each collector

Benchmarks will allow for easier identification of slow collectors.
Additionally, they increase test coverage of the collectors, with some
collectors now reaching 80-95% coverage with this change.

Collector benchmarks have been structed so that common functionality is
present in `collector/collector_test.go` as is done with non-test
functionality in `collector/collector.go`.
Test logic that is specific to individual collectors is present in the
collector test file (E.G. `collector/process_test.go` for the Process
collector).

Signed-off-by: Ben Reedy <breed808@breed808.com>

collector/collector_test.go

@@ -3,6 +3,8 @@ package collector
 import (
 	"reflect"
 	"testing"
+
+	"github.com/prometheus/client_golang/prometheus"
 )
 
 func TestExpandChildCollectors(t *testing.T) {
@@ -32,3 +34,27 @@ func TestExpandChildCollectors(t *testing.T) {
 		})
 	}
 }
+
+func benchmarkCollector(b *testing.B, name string, collectFunc func() (Collector, error)) {
+	// Create perflib scrape context. Some perflib collectors required a correct context,
+	// or will fail during benchmark.
+	scrapeContext, err := PrepareScrapeContext([]string{name})
+	if err != nil {
+		b.Error(err)
+	}
+	c, err := collectFunc()
+	if err != nil {
+		b.Error(err)
+	}
+
+	metrics := make(chan prometheus.Metric)
+	go func() {
+		for {
+			<-metrics
+		}
+	}()
+
+	for i := 0; i < b.N; i++ {
+		c.Collect(scrapeContext, metrics)
+	}
+}