Add gateway metrics

[eliteprox]

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

This change adds roundtrip time metrics for all pipelines, logging the specific pipeline, model and roundtrip time.

You can visualize this with a timeseries chart using the following query:

rate(livepeer_ai_roundtrip_time_seconds_sum[1m]) / rate(livepeer_ai_roundtrip_time_seconds_count[1m])

Use the following variable for the Legend: {{pipeline}}, {{model_name}}

Result:

Specific updates (required)

• Updates census.go

How did you test each of these updates (required)

Does this pull request close any open issues?

Checklist:

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

[rickstaa]

Replaced by https://github.com/livepeer/go-livepeer/pull/3087.