internal/telemetry/metrics: document concurrently using (#891)

Document that metricRegistry is not safe for concurrently use. While at
it, remove t.Parallel() in tests which use metricRegistry, which causes
data race, caught by:

	go test -race ./internal/telemetry/metrics

internal/telemetry/metrics/registry.go

@@ -18,6 +18,8 @@ var (
 // metricRegistry holds the non-view metrics and handles safe
 // initialization and updates.  Behavior without using newMetricRegistry()
 // is undefined.
+//
+// It is not safe to use metricRegistry concurrently.
 type metricRegistry struct {
 	registry       *metric.Registry
 	buildInfo      *metric.Int64Gauge