Initialize the labels map in the response.

[qingling128]

Fixes the following error when the labels is not initialized:

Container logs:
level=info ts=2020-03-02T03:17:42.590Z caller=main.go:293 msg="Starting Stackdriver Prometheus sidecar" version="(version=0.7.3, branch=master, revision=2548e8a7b383adc9217afe8570247ea84e3ef2c0)"
level=info ts=2020-03-02T03:17:42.591Z caller=main.go:294 build_context="(go=go1.12, user=kbuilder@kokoro-gcp-ubuntu-prod-40088225, date=20200206-15:24:17)"
level=info ts=2020-03-02T03:17:42.591Z caller=main.go:295 host_details="(Linux 4.14.138+ #1 SMP Tue Sep 3 02:58:08 PDT 2019 x86_64 apigee-metrics-v111-fd6fj (none))"
level=info ts=2020-03-02T03:17:42.592Z caller=main.go:296 fd_limits="(soft=1048576, hard=1048576)"
panic: assignment to entry in nil map

Labels can be optionally extracted from OC_RESOURCE_TYPE and OC_RESOURCE_LABELS env vars (https://github.com/census-instrumentation/opencensus-go/blob/1901b56b9515b0c34f5d25a5bce982dfc543d64b/resource/resource.go#L30). In case those envs are not set, labels will not be initialized: https://github.com/census-instrumentation/opencensus-go/blob/1901b56b9515b0c34f5d25a5bce982dfc543d64b/resource/resource.go#L98

For the case here, we are in fact sending metrics to Stackdriver. So OC_RESOURCE_TYPE and OC_RESOURCE_LABELS may not be needed. But we should init the Labels field.

I'll send an equivalent PR to the upstream repo as well.

[quentinmit]

You can't make changes to the vendor directory like that; modern Go tooling will overwrite/ignore it. You need to put this upstream, or fork the upstream repo and change go.mod to point to the fork.

[qingling128]

The upstream PR has been approved and merged: census-ecosystem/opencensus-go-exporter-stackdriver#264.

[qingling128]

Hi, when is the next release gonna happen? Any rough ETA?

[tsekine354]

Please reopen this issue. As mentioned in #238 (comment) somehow, go.mod should be updated for contrib.go.opencensus.io/exporter/stackdriver. It is still pointing v0.12.8, but should be v0.13.2 (or higher).

[qingling128]

Oh, sorry. The Hi, when is the next release gonna happen? Any rough ETA? is meant for the other PR in upstream (census-ecosystem/opencensus-go-exporter-stackdriver#264). I put it in the wrong PR. This current one is already closed.