feat(ai): add AI gateway metrics

[rickstaa]

What does this pull request do? Explain your changes. (required)

This pull request introduces new Gateway AI metrics to the ai-video branch:

• Ticket_value_sent: The value of the AI tickets that are send to the Orchestrators.
• Tickets_send: The total amount of tickets send to the Orchestrators.
• ai_models_requested: Tracks the number of requests per capability and model.
• ai_request_latency_score: Measures latency scores per model request per orchestrator.
• ai_request_price: Records the price paid per unit for each model request.
• ai_request_errors: Logs AI request errors per orchestrator. If no orchestrator is specified, this indicates a general request scenario where no orchestrators were found.

Specific updates (required)

• Updates census.go to include the new Gateway metrics.
• Updates ai_process to log these metrics.

How did you test each of these updates (required)

I set up both an on-chain and off-chain gateway to validate the metrics. I verified their visibility at http://localhost:5935/metrics and ensured they were correctly visualized in Grafana.

Does this pull request close any open issues?

This implements the functionality outlined in https://livepeer-ai.productlane.com/roadmap?id=58c9cd5d-1c64-4fb3-b8d0-a7e20b7865a2.

Checklist:

• [x] Read the contribution guide
• [x] make runs successfully
• [ ] All tests in ./test.sh pass
• [x] README and other documentation updated
• [ ] Pending changelog updated

How to test

1. Check out this pull request.
2. Spin up an on-chain gateway with attached orchestrators.
3. Clone the repository https://github.com/rickstaa/livepeer-monitor-test.
4. Execute the Dockerfile in that repository to launch Prometheus and Grafana servers.
5. Navigate to http://localhost:5935/metrics to view the new AI Gateway metrics.
6. Visit http://localhost:3000 to inspect these metrics in Grafana.

[pwilczynskiclearcode]

https://github.com/livepeer/go-livepeer/issues/3185