Eventing metrics

[nachocano]

What is changing? (Please include as many details as possible.)

The current eventing metrics have changed, and we need to properly document them in knative/docs.
We also need to add instructions on how to configure the different metrics backends.

How will this impact our users?

Users will be able to use metrics to better understand what is going on in their clusters.

In what release will this take happen (to the best of your knowledge)?

v0.9

Context

Link to associated PRs or issues from other repos here.

1. E2E Metrics for Importer/Broker/Trigger model eventing#1693
2. Some initial work here: https://github.com/knative/eventing/blob/master/docs/metrics.md

[samodell]

/lifecycle frozen