Implement Advanced Filtering proposal

[grantr]

Implements #1047 at 76554b8.

Proposed Changes

• Add attributes filter to the Trigger spec accepting a map of attribute keys and values to compare to the event context attributes, including extensions. Replaces sourceAndType.
• Add expression filter to the Trigger spec accepting a CEL expression to be evaluated with respect to the event context and data.
• Deprecate sourceAndType filter. This filter may be removed or translated into an attributes filter in a future release.
• Validate that only a single filter strategy is selected per Trigger.

Details

Only one filter can be specified per Trigger. The Trigger will be rejected as invalid if multiple filters are specified. If no filters are specified, that's currently interpreted as a filter matching everything.

CEL expressions are compiled into programs and cached in an LRU cache by expression string. Currently the cache size is fixed at 100 entries, though that should eventually be based on the number of Triggers.

Expressions are inspected to determine if they rely on parsing event data. Event data is not parsed when the expression doesn't need it.

The SourceAndType and Attributes filters are implemented by translation to CEL expression.

Attribute filter keys containing special characters such as . are supported.

Attribute filters do not support comparing non-string values. Attribute filters containing non-string values will be TODO. Non-string attribute values compared to attribute filters will generate an evaluation error.

Evaluation errors cause events to be rejected.

Remaining work

Currently evaluation errors are only surfaced in dispatcher logs. This should be surfaced in a form more visible to the user.
Instrumentation is missing and should be added before release.

Release Note

- Add `attributes` filter to the Trigger spec accepting a map of attribute keys and values to compare to the event context attributes, including extensions. Replaces `sourceAndType`.
- Add `expression` filter to the Trigger spec accepting a CEL expression to be evaluated with respect to the event context and data.
- Deprecate `sourceAndType` filter. This filter may be removed or translated into an `attributes` filter in a future release.
- Validate that only a single filter strategy is selected per Trigger.

/hold
Holding for Docs PR and #1047 merge.

Fixes #930.

[knative-prow-robot]

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[grantr]

The receiver now instruments filtering. This is ready for review.

[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/apis/eventing/v1alpha1/trigger_validation.go	100.0%	97.4%	-2.6
pkg/broker/expression_filter.go	Do not exist	87.2%
pkg/broker/metrics.go	Do not exist	71.4%
pkg/broker/program_cache.go	Do not exist	83.3%
pkg/broker/receiver.go	82.9%	87.3%	4.3
pkg/reconciler/trigger/trigger.go	Do not exist	83.4%

[grantr]

This is on hold for now; see #1047 (comment).

[grantr]

Attributes filters made it in via #1643.