Add health endpoint to event_display server

[antoineco]

Proposed Changes

• 🎁 Adds a health HTTP handler to event_display on the /healthz endpoint.

This handler responds with "204 No Content" to any request, regardless of the HTTP method. It is meant to be used for probing liveness/readiness.

Background: I observed race conditions on several occasions downstream, in e2e tests that rely on event_display. E.g. an event is sent as soon as the Pod starts, but the CE client isn't started yet 🤷.

I purposely avoided using something from k8s/knative to avoid large imports ("fat binary").

Pre-review Checklist

• At least 80% unit test coverage (⚠️ event_display didn't have unit tests prior to this PR. I'll add more tests in a subsequent refactoring.)
• E2E tests for any new behavior
• Docs PR for any user-facing impact
• Spec PR for any new API feature
• Conformance test for any change to the spec

Release Note

Add a health endpoint to `event_display` for enabling readiness probes.

Docs

[knative-prow-robot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[codecov]

Codecov Report

Merging #5608 (585c8c7) into main (7251c1b) will decrease coverage by 0.06%.
The diff coverage is 58.33%.

@@            Coverage Diff             @@
##             main    #5608      +/-   ##
==========================================
- Coverage   82.79%   82.73%   -0.07%     
==========================================
  Files         199      200       +1     
  Lines        6230     6243      +13     
==========================================
+ Hits         5158     5165       +7     
- Misses        744      748       +4     
- Partials      328      330       +2     

Impacted Files	Coverage Δ
cmd/event_display/main.go	53.84% <58.33%> (ø)

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[antoineco]

Demo:

$ curl -D- http://localhost:8080/ -H "Ce-Specversion: 1.0" -H "Ce-Id: 0001" -H "Ce-Type: greeting" -H "Ce-Source: curl" -H "Content-Type: application/json" -d '{"msg":"Hello, World!"}
HTTP/1.1 200 OK
Date: Sat, 24 Jul 2021 14:33:09 GMT
Content-Length: 0

$ curl -D- http://localhost:8080/healthz
HTTP/1.1 204 No Content
Date: Sat, 24 Jul 2021 14:31:48 GMT

[knative-metrics-robot]

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

File	Old Coverage	New Coverage	Delta
cmd/event_display/main.go	Do not exist	63.6%

[matzew]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: antoineco, matzew

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• OWNERS [antoineco,matzew]

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[antoineco]

@n3wscott I needed something quick so I used a Middleware for the time being, but this feels clumsy to me (see the implementation of healthzMiddleware() below which is especially hacky).

Since I'm going to refactor this for testability, I thought now was a good time to ask for your expertise :)