internal/metrics: fix leaking stackdriver exporter
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 <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Auto-Submit: Emmanuel Odeke <bd234ba4276433f0e5fc7a8fa2d18274fa711567@orijtech.com>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Run-TryBot: Emmanuel Odeke <bd234ba4276433f0e5fc7a8fa2d18274fa711567@orijtech.com>
Reviewed-by: Joedian Reid <454f26308bdf67a957daa60c4ad294a9b7c3fcbc@golang.org>

golang/playground,golang/playground,golang/playground