internal/metrics: fix leaking stackdriver exporter

Found internally at Orijtech, Inc by StaticMajor, this
change invokes .Close() on the Stackdriver Metrics Exporter
if its .StartMetricsExporter() method returns an error.

Change-Id: Ib17cfdd7c1f4f8bcde6e08372de3a4f1c367575d
Reviewed-on: https://go-review.googlesource.com/c/playground/+/442715
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Joedian Reid <joedian@golang.org>

internal/metrics/service.go

diff --git a/internal/metrics/service.go b/internal/metrics/service.go
index f6126dd..a19aa26 100644
--- a/internal/metrics/service.go
+++ b/internal/metrics/service.go
@@ -66,6 +66,7 @@
 	view.SetReportingPeriod(time.Minute)
 	// Start the metrics exporter.
 	if err := sde.StartMetricsExporter(); err != nil {
+		sde.Close()
 		return nil, err
 	}