metrics: restore global registry in unit tests (#5399)

Currently there appears to be a test order dependency between a couple
of the info_test.go test cases and the Test_PrometheusHandler test. This
can be exposed by running:

  go test -count 2 ./internal/telemetry/metrics

The test cases in info_test.go overwrite the global 'registry' variable,
which seems to prevent Test_PrometheusHandler from being able to export
the internal Go metrics. Add a helper method to restore the original
registry after these test cases.

internal/telemetry/metrics/info_test.go

@@ -65,7 +65,7 @@ func Test_SetDBConfigInfo(t *testing.T) {
 }
 
 func Test_SetBuildInfo(t *testing.T) {
-	registry = newMetricRegistry()
+	initTemporaryMetricsRegistry(t)
 
 	version.Version = "v0.0.1"
 	version.GitCommit = "deadbeef"
@@ -84,7 +84,7 @@ func Test_SetBuildInfo(t *testing.T) {
 }
 
 func Test_AddPolicyCountCallback(t *testing.T) {
-	registry = newMetricRegistry()
+	initTemporaryMetricsRegistry(t)
 
 	wantValue := int64(42)
 	wantLabels := []metricdata.LabelValue{
@@ -106,3 +106,9 @@ func Test_RegisterInfoMetrics(t *testing.T) {
 		t.Error("Did not find enough registries")
 	}
 }
+
+func initTemporaryMetricsRegistry(t *testing.T) {
+	original := registry
+	registry = newMetricRegistry()
+	t.Cleanup(func() { registry = original })
+}