Expose metrics defined by the spec in Prometheus format

[pierDipi]

This PR exposes the following metrics:

1. http_events_sent_total
2. http_requests_produce_total
3. http_requests_malformed_total

All of the above metrics are counters.

(1) is incremented when the dispatcher commits the offset to Kafka.
(2) is incremented when the receiver sends successfully the message to Kafka.
(3) is incremented when the receiver receives a malformed event.

Signed-off-by Pierangelo Di Pilato pierangelodipilato@gmail.com

Fixes #227

Proposed Changes

• Expose metrics defined by the spec in Prometheus format

Prometheus Dashboard

Release Note

- 🎁 Add new feature
The receiver component exposes:
  - http_requests_produce_total - Number of accepted produce requests (200-level responses)
  - http_requests_malformed_total - Number of malformed produce requests (400-level responses)
The dispatcher component exposes:
  - http_events_sent_total - Number of events delivered

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[codecov]

Codecov Report

Merging #244 into master will decrease coverage by 0.10%.
The diff coverage is 65.38%.

@@             Coverage Diff              @@
##             master     #244      +/-   ##
============================================
- Coverage     76.40%   76.30%   -0.11%     
  Complexity      170      170              
============================================
  Files            49       49              
  Lines          1420     1435      +15     
  Branches         46       46              
============================================
+ Hits           1085     1095      +10     
- Misses          266      271       +5     
  Partials         69       69              

Flag	Coverage Δ	Complexity Δ
#java-unittests	76.05% <65.38%> (-0.21%)	170.00 <4.00> (ø)
#unittests	76.30% <65.38%> (-0.11%)	170.00 <4.00> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ	Complexity Δ
...knative/eventing/kafka/broker/dispatcher/Main.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...v/knative/eventing/kafka/broker/receiver/Main.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...ka/broker/core/metrics/MetricsOptionsProvider.java	90.00% <100.00%> (+1.11%)	1.00 <0.00> (ø)
...ispatcher/ConsumerRecordOffsetStrategyFactory.java	100.00% <100.00%> (ø)	1.00 <1.00> (ø)
...trategy/UnorderedConsumerRecordOffsetStrategy.java	90.32% <100.00%> (+1.86%)	9.00 <2.00> (ø)
...eventing/kafka/broker/receiver/RequestHandler.java	95.29% <100.00%> (+0.23%)	14.00 <1.00> (ø)

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[slinkydeveloper]

Nice trick reusing the other tests ;)
/lgtm