fix: cache promise instead of ProjectId

[JustinBeckwith]

Addresses #187

[ofrobots]

This harmless looking change will break context propagation. For some background see this code and the corresponding issue in google-auto-auth.

Speaking of which; google-auto-auth already caches the auth client and project id. Such logic / automation belongs in that package, not here.

[JustinBeckwith]

The way this code works today, it can be kind of a bad bug. Take a look at #187 - we're queuing up thousands of calls, waiting for the initial projectId to populate. Generally - I'd say it's more important to fix the bug than to make it work with Stackdriver (totally willing to debate it of course).

On the discussion of this library vs google-auto-auth - I view that library as a band aid, because this one was basically abandoned. I would like to reach a place where it isn't needed anymore, but it is going to take time.

If we wanted to cache the promise, and not break context propagation - what can I do?

[ofrobots]

Generally - I'd say it's more important to fix the bug than to make it work with Stackdriver

It is not about Stackdriver. This is about breaking long stack traces, NewRelic and all other async debugging tools out this. This module is the basis of all @google-cloud/ modules which means that all of them will stop working with these debug / tracing tools.

If we wanted to cache the promise, and not break context propagation - what can I do?

Similar to what was done in stephenplusplus/google-auto-auth#21. Basically, use the promise as a queue while it is pending. Once resolved, cache the value too, and stop using the promise.

[JustinBeckwith]

I think I got it :) @ofrobots can you take another look?