Stackdriver exporters: automatically determine project_id

[g-easy]

When running on GCE, this should be doable via the metadata server: https://cloud.google.com/compute/docs/storing-retrieving-metadata#default

[mandarjog]

/subscribe

[alichnewsky]

sample implementation in a test program here. Uses cpr as an idiomatic wrapper to libcurl.

• needs to be encapsulated, moved to a library ( in opencensus/common/internal/cloud ? ). Unless a dedicated, idiomatic library is provided by google-cloud-cpp.
• needs to make the distinction between GCE VM, GKE container/pod, ( a docker container inside GCE VM? inside AppEngine flex ? inside Cloud Run ? they will have different well known google.api.MonitoredResource objects and labels...)
• a lingering question remains in my mind on the difference between the project id at the monitored resource level and as a monitored resource label (lines 210-213) . Are they the same when on GCE VM? if a workspace monitors multiple project, does the code submitting the metric need to know a 'stackdriver project id ?'