feat(js): Enable Firebase Genkit Monitoring via an environment variable

[schnecle]

Allow developers to turn Firebase Genkit Monitoring on and off via environment variables.

Checklist (if applicable):

• [x] PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• [x] Tested (manually, unit tested, etc.)
• [ ] Docs updated (updated docs or a docs bug required)

Docs will need to be updated in a subsequent PR.