metricsgen: port metrics code generation tool (#9156)

This is a port of #8470 and #8501 from the former mainline branch, in
support of #2600 and #9076. 

No additional changes other than checking out the code have been
implemented in this PR. A subsequent PR will incorporate this code.

scripts/metricsgen/testdata/basic/metrics.go

@@ -0,0 +1,11 @@
+package basic
+
+import "github.com/go-kit/kit/metrics"
+
+//go:generate go run ../../../../scripts/metricsgen -struct=Metrics
+
+// Metrics contains metrics exposed by this package.
+type Metrics struct {
+	// simple metric that tracks the height of the chain.
+	Height metrics.Gauge
+}