create Google app creds file from env var as needed

[groovecoder]

This PR fixes MPP-3467.

It changes the profiling code to automatically create the necessary GCP key JSON file from the value of the GOOGLE_CLOUD_PROFILER_CREDENTIALS_B64 environment variable. This should work identically across local, dev, stage, and prod environments.

How to test:

1. Follow the doc!
2. When you runserver you should see output lines: HTTPAdapter.get_connection url: http://metadata/computeMetadata/v1/instance/zone which indicates that the profiler code is working

• l10n changes have been submitted to the l10n repository, if any.
• I've added a unit test to test for potential regressions of this bug.
• I've added or updated relevant docs in the docs/ directory.
• All UI revisions follow the coding standards, and use Protocol tokens where applicable (see /frontend/src/styles/tokens.scss).
• Commits in this PR are minimal and have descriptive commit messages.

[jwhitlock]

Thanks @groovecoder, this looks a little safer. Please consider what should happen if GOOGLE_APPLICATION_CREDENTIALS is a directory - raise an exception, or skip loading the profiler?

[jwhitlock]

If you set GOOGLE_APPLICATION_CREDENTIALS to "privaterelay", this fails with IsADirectoryError: [Errno 21] Is a directory: 'privaterelay'. I think the line should be:

Suggested change

	if not gcp_key_json_path.exists() or not gcp_key_json_path.is_file():
	if not gcp_key_json_path.exists() and gcp_key_json_path.is_file():

At the same time, you'll get a IsADirectoryError in a couple of lines when you try to read it. So, maybe drop the is_file() check and let Relay crash on startup.

[groovecoder]

I removed the is_file check and expanded the try/except block around the code for the file so that the Relay app can still start up and run even if google cloud profiler can't.

[jwhitlock]

Thanks, this worked with my tests (folder, empty json). One suggestion you could take or leave.

[jwhitlock]

Suggestion: print more of the error:

Suggested change

	except:
	print(
	f"exception while starting google cloud profiler with key file: {gcp_key_json_path}"
	)
	except Exception as exc:
	print(
	f"exception {repr(exc)}"
	" while starting google cloud profiler"
	f" with key file: {gcp_key_json_path}"
	)