Add support for Google's AppEngine Standard

[bogdandrutu]

Add a contrib package/artifact that allows users to interact with AppEngine trace context.

See something similar here: https://github.com/GoogleCloudPlatform/cloud-trace-java/tree/master/services/appengine-standard-service

What should be provided:

• Encode/Decode to "com.google.apphosting.api.CloudTraceContext".
• Interaction with the appengine context "com.google.apphosting.api.CloudTrace". This may be hard because the opencensus uses grpc.Context and may require some non trivial changes.

[sebright2]

/cc @dinooliva

[bogdandrutu]

The interaction with the com.google.apphosting.api.CloudTrace can be done via io.grpc.Context.Storage. Investigating that option now.

[songy23]

Should this issue be closed via https://github.com/census-instrumentation/opencensus-java/pull/1214?

[sebright2]

#1214 only implemented the first bullet point.

[jaycarlton]

Hi, I'm not sure if this is the place to ask this, but is OpenCensus supported in AppEngine Standard for stats monitoring? I saw this page, but wasn't sure if it was up-to-date.

I've been using it for a while in that environment with partial success. Thanks.