fix(go): fixes to AIM telemetry

[huangjeff5]

Addresses feedback here: https://docs.google.com/spreadsheets/d/1yYuyBLzA6S9blsGv05C-YKJSqyZsfMjh8IX7SW4RbTY/edit?gid=0#gid=0

Mainly:

1. Move the generate span generation to the ai.Generate() method - the Generation Action already gets a span from the action framework so there's no need to add it there, and ai.Generate() DOES need the extra span since it's not an action. This is the cause of most of the weirdness
2. Add support for GCLOUD_PROJECT (same as genkit ts)
3. Make firebase plugin match googlecloud plugin in terms of options (pass thru directly)
4. Handle credential detection automatically
5. Make sure exact same logs as genkit ts are being emitted by ensuring that logic for which telemetry module to call is exactly the same. Also update the user engagement tests to reflect reality; since user engagement spans have type="userEngagement"

Tested manually, some screenshots:

[schnecle]

Looks good from a monitoring perspective