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/cs_test.go

@@ -2,22 +2,8 @@ package collector
 
 import (
 	"testing"
-
-	"github.com/prometheus/client_golang/prometheus"
 )
 
-func BenchmarkCsCollect(b *testing.B) {
-	c, err := NewCSCollector()
-	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)
-	}
+func BenchmarkCsCollector(b *testing.B) {
+	benchmarkCollector(b, "cs", NewCSCollector)
 }