firestore: enable a flag when emulator is used

[raymonstah]

Is your feature request related to a problem? Please describe.
Given a firestore client, I'd like to be able to tell if the client is connected to the actual firestore servers or just the emulator.

Describe the solution you'd like
A bool on the client or a method to indicate if the client is connected to an emulator or not.

Describe alternatives you've considered
No alternatives considered.

Additional context
Happy to submit a PR for this.

[bhshkh]

A client will connect to emulator only when FIRESTORE_EMULATOR_HOST environment variable is set.

The same condition can be used.

Please provide more information why a separate variable is required and the above env variable cannot be used

[raymonstah]

Hey @bhshkh,

Thanks for getting back to me. I understand that when the environment variable is set, the client will connect to the emulator.

However, when working with multiple clients, it would be useful to know which client is connected to the real firestore servers, and which client is connected to the emulator. This could be useful as a safeguard so we aren't accidentally deleting data from "production" when we meant to be doing it in the emulator environment.

However, going back to your original comment, it can be awkward to set up clients that are using both an emulator and connected to the real servers, since it involves programmatically setting and unsetting the environment variable:

os.SetEnv("FIRESTORE_EMULATOR_HOST", "localhost")
// set up emulator client
os.UnsetEnv("FIRESTORE_EMULATOR_HOST")
// set up production client

[bhshkh]

@raymonstah please submit a PR if you can. I'll be happy to review it

[raymonstah]

@bhshkh #11359