Use the runtime/metrics package for the Go collector for 1.17+

This change introduces use of the runtime/metrics package in place of
runtime.MemStats for Go 1.17 or later. The runtime/metrics package was
introduced in Go 1.16, but not all the old metrics were accounted for
until 1.17.

The runtime/metrics package offers several advantages over using
runtime.MemStats:
* The list of metrics and their descriptions are machine-readable,
  allowing new metrics to get added without any additional work.
* Detailed histogram-based metrics are now available, offering much
  deeper insights into the Go runtime.
* The runtime/metrics API is significantly more efficient than
  runtime.MemStats, even with the additional metrics added, because
  it does not require any stop-the-world events.

That being said, integrating the package comes with some caveats, some
of which were discussed in #842. Namely:
* The old MemStats-based metrics need to continue working, so they're
  exported under their old names backed by equivalent runtime/metrics
  metrics.
* Earlier versions of Go need to continue working, so the old code
  remains, but behind a build tag.

Finally, a few notes about the implementation:
* This change includes a whole bunch of refactoring to avoid significant
  code duplication.
* This change adds a new histogram metric type specifically optimized
  for runtime/metrics histograms. This type's methods also include
  additional logic to deal with differences in bounds conventions.
* This change makes a whole bunch of decisions about how runtime/metrics
  names are translated.

Signed-off-by: Michael Anthony Knyszek <mknyszek@google.com>

prometheus/build_info_collector.go

@@ -0,0 +1,38 @@
+// Copyright 2021 The Prometheus Authors
+// 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 prometheus
+
+import "runtime/debug"
+
+// NewBuildInfoCollector is the obsolete version of collectors.NewBuildInfoCollector.
+// See there for documentation.
+//
+// Deprecated: Use collectors.NewBuildInfoCollector instead.
+func NewBuildInfoCollector() Collector {
+	path, version, sum := "unknown", "unknown", "unknown"
+	if bi, ok := debug.ReadBuildInfo(); ok {
+		path = bi.Main.Path
+		version = bi.Main.Version
+		sum = bi.Main.Sum
+	}
+	c := &selfCollector{MustNewConstMetric(
+		NewDesc(
+			"go_build_info",
+			"Build information about the main Go module.",
+			nil, Labels{"path": path, "version": version, "checksum": sum},
+		),
+		GaugeValue, 1)}
+	c.init(c.self)
+	return c
+}