Broker metrics refactor

[nachocano]

Helps with #1693

Proposed Changes

• Renaming metrics to match serving naming.
• Add stats reporters to broker ingress and broker filter.
• Added more filtering labels to metrics.
• Reporting delivery times only to things that are dispatched (e.g., not if the subscriberURI was invalid, or if they didn't pass the filter). This is still subject to change

Release Note

Broker and Trigger metrics have been renamed. Please refer to the docs for description of the new metrics.

[nachocano]

/assign @Harwayne
/assign @sayanh

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/assign @Harwayne
/assign @sayanh

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[nachocano]

@capri-xiyue this refactor should also fix #1423

[nachocano]

@Harwayne this is ready for another look. Thanks for the comments, all made sense!

[knative-metrics-robot]

The following is the coverage report on pkg/.
Say /test pull-knative-eventing-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/broker/filter/filter_handler.go	82.2%	84.1%	1.9
pkg/broker/filter/stats_reporter.go	Do not exist	78.9%
pkg/broker/ingress/ingress_handler.go	60.7%	56.2%	-4.5
pkg/broker/ingress/stats_reporter.go	Do not exist	78.1%
pkg/broker/metrics.go	50.0%	57.1%	7.1

[Harwayne]

/lgtm
/approve

Letting this submit as-is, because others are going to use it as a template and the only remaining comment is fairly minor and does not affect what will be emulated.

[Harwayne]

if r.filterEventByAttributes(ctx, attrs, event) {
    return passFilter
}
return filterResult

Even better is to have filterEventByAttributes return the filterResult.

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Harwayne, nachocano

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