workload: add prometheus NewGoCollector

Previously, the workload would register only a Histogram
for the corresponding workload. This change adds NewGoCollector
which exports Go runtime metrics, i.e., gc and goroutines.

This change makes it easy to investigate performance
regressions within the workload generator.

Epic: none

Release note: workload generators now export Go runtime metrics
via Prometheus endpoint.

pkg/workload/cli/BUILD.bazel

@@ -28,6 +28,7 @@ go_library(
         "//pkg/workload/histogram",
         "//pkg/workload/workloadsql",
         "@com_github_cockroachdb_errors//:errors",
+        "@com_github_prometheus_client_golang//prometheus/collectors",
         "@com_github_prometheus_client_golang//prometheus/promhttp",
         "@com_github_spf13_cobra//:cobra",
         "@com_github_spf13_pflag//:pflag",

pkg/workload/cli/run.go

@@ -37,6 +37,7 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/workload/histogram"
 	"github.com/cockroachdb/cockroach/pkg/workload/workloadsql"
 	"github.com/cockroachdb/errors"
+	"github.com/prometheus/client_golang/prometheus/collectors"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"github.com/spf13/cobra"
 	"github.com/spf13/pflag"
@@ -415,6 +416,7 @@ func runRun(gen workload.Generator, urls []string, dbName string) error {
 		*histogramsMaxLatency,
 		gen.Meta().Name,
 	)
+	reg.Registerer().MustRegister(collectors.NewGoCollector())
 	// Expose the prometheus gatherer.
 	go func() {
 		if err := http.ListenAndServe(