Adding optional switch to capture project ID in from_service_account_json(). - #3436

It would be great to solve this by just consolidating ClientWithProject and ClientWithCredentials Client

@jonparrott See the table I made about usage. The only issue with that is some APIs don't use a project (and we don't want to fail their constructors over a missing project).

@jonparrott Mostly it doesn't help because it isn't true?

Or do you mean something else?

But there is no google.auth.default_project?

Makes sense.

@lukesneeringer WDYT of us just using project on every Client instance? (This would just bring Client and ClientWithProject together, ClientWithProject is already a tiny impl.)

FTR, ClientWithProject is skipped by the following:

$ for f in $(ls */google/cloud/*/client.py); do
       if ! grep -q ClientWithProject $f; then
           echo $f
       fi
  done
bigtable/google/cloud/bigtable/client.py
language/google/cloud/language/client.py
resource_manager/google/cloud/resource_manager/client.py
spanner/google/cloud/spanner/client.py
speech/google/cloud/speech/client.py
translate/google/cloud/translate/client.py

Of those, the following have _ClientProjectMixin included:

$ for f in $(ls */google/cloud/*/client.py); do
       if ! grep -q ClientWithProject $f; then
           echo $f
       fi
  done | xargs grep -l _ClientProjectMixin
bigtable/google/cloud/bigtable/client.py
spanner/google/cloud/spanner/client.py

@tseaver This is (intended to be) captured in my table

@tseaver Sorry I missed your LGTM 18 days ago.

It's good I got this in, in case we have a google-cloud-core release for your gRPC remap PR, this can go in too.