Introduce optional profiling through Stackdriver Profiler

This introduces the first sample profiler driver (Stackdriver), allowing initializing any Go service that uses labkit (and upgrades to the new version) with the GITLAB_CONTINUOUS_PROFILING env variable as such:

GITLAB_CONTINUOUS_PROFILING="stackdriver?service=gitaly&service_version=1.0.1&project_id=test-123" go run file.go

Passing GITLAB_CONTINUOUS_PROFILING=stackdriver (without further params) will make the Stackdriver profiler guess these values based certain environment criterias described at https://godoc.org/cloud.google.com/go/profiler#Config.

In order to use the profiler, users of labkit will need to compile it with the -tags continuous_profiler_stackdriver, otherwise no dependencies will be added and GITLAB_CONTINUOUS_PROFILING will take no effect.

Binary size comparison with and without the tag

2.6.3 in gitlab-workhorse/ on master
› ls -l gitlab-workhorse
-rwxr-xr-x 1 osw staff 23300020 Feb  7 17:42 gitlab-workhorse*

2.6.3 in gitlab-workhorse/ on master
› fg
[1]  + continued  nvim .

2.6.3 in gitlab-workhorse/ on master
› make
### clean-build
rm -rf /Users/osw/projects/gitlab/gitlab-workhorse/_build
### Setting up target directory
rm -rf "/Users/osw/projects/gitlab/gitlab-workhorse/_build"
mkdir -p "/Users/osw/projects/gitlab/gitlab-workhorse/_build"
touch "/Users/osw/projects/gitlab/gitlab-workhorse/_build/.ok"
### Building gitlab-zip-cat
go build -ldflags "-X main.Version=v8.20.0 -X main.BuildTime=20200207.204449" -tags "tracer_static tracer_static_jaeger continuous_profiler_stackdriver" -o /Users/osw/projects/gitlab/gitlab-workhorse/gitlab-zip-cat gitlab.com/gitlab-org/gitlab-workhorse/cmd/gitlab-zip-cat
### Building gitlab-zip-metadata
go build -ldflags "-X main.Version=v8.20.0 -X main.BuildTime=20200207.204449" -tags "tracer_static tracer_static_jaeger continuous_profiler_stackdriver" -o /Users/osw/projects/gitlab/gitlab-workhorse/gitlab-zip-metadata gitlab.com/gitlab-org/gitlab-workhorse/cmd/gitlab-zip-metadata
### Building gitlab-workhorse
go build -ldflags "-X main.Version=v8.20.0 -X main.BuildTime=20200207.204449" -tags "tracer_static tracer_static_jaeger continuous_profiler_stackdriver" -o /Users/osw/projects/gitlab/gitlab-workhorse/gitlab-workhorse gitlab.com/gitlab-org/gitlab-workhorse

2.6.3 in gitlab-workhorse/ on master
› ls -l gitlab-workhorse
-rwxr-xr-x 1 osw staff 25092772 Feb  7 17:44 gitlab-workhorse*

So it adds about 1.7MB to the workhorse binary, only when adding -tags continuous_profiler_stackdriver in the compilation process.

gitlab-com/gl-infra/scalability#101 (closed)

Closes #8 (closed)