sandbox: instrument HTTP handlers with StackDriver

This change adds OpenCensus's HTTP instrumentation to the sandbox. In
development mode, it exposes a prometheus metrics interface on /statusz.
This is the first in a series of CLs to add instrumentation to different
parts of the sandbox to help investigate instability issues. For now,
reporting metrics around our responses per handler to StackDriver will
be helpful.

OpenTelemetry would be preferable, as it is the successor of OpenCensus,
however the StackDriver integration is not quite done.

Updates golang/go#25224
Updates golang/go#38530

Change-Id: I600fd695bb66c8bee16bc0b778d51930f4cdd476
Reviewed-on: https://go-review.googlesource.com/c/playground/+/229679
Run-TryBot: Alexander Rakoczy <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>

Author: toothrot

go.mod

@@ -3,12 +3,20 @@ module golang.org/x/playground
 go 1.12
 
 require (
-	cloud.google.com/go v0.38.0
+	cloud.google.com/go v0.54.0
+	cloud.google.com/go/datastore v1.1.0
+	cloud.google.com/go/pubsub v1.3.1 // indirect
+	contrib.go.opencensus.io/exporter/prometheus v0.1.0
+	contrib.go.opencensus.io/exporter/stackdriver v0.13.1
 	github.com/bradfitz/gomemcache v0.0.0-20190329173943-551aad21a668
-	github.com/google/go-cmp v0.3.0
+	github.com/google/go-cmp v0.4.0
+	go.opencensus.io v0.22.3
 	golang.org/x/build v0.0.0-20190709001953-30c0e6b89ea0
 	golang.org/x/mod v0.2.0
 	golang.org/x/tools v0.0.0-20200420001825-978e26b7c37c
-	google.golang.org/api v0.4.0
+	google.golang.org/api v0.20.0
+	google.golang.org/genproto v0.0.0-20200312145019-da6875a35672
+	gopkg.in/check.v1 v1.0.0-20190902080502-41f04d3bba15 // indirect
+	gopkg.in/yaml.v2 v2.2.7 // indirect
 	grpc.go4.org v0.0.0-20170609214715-11d0a25b4919
 )