Simple PubSub Buffer

[bbengfort]

Scope of changes

Implements a simple channel based, in-memory pub/sub buffer. If there are no subscribers the events are dropped.

Fixes SC-9272

Type of change

• new feature
• bug fix
• documentation
• testing
• technical debt
• other (describe)

Acceptance criteria

To run the ensign server with monitoring:

$ ./containers/build.sh build 
$ ./containers/build.sh up

This will run ensign, prometheus, and grafana in docker compose

You can then run event generators as follows:

go run ./cmd/debug generate

You can run multiple of these in different terminals; the IDs should be monotonically increasing

To run consumers, use:

go run ./cmd/debug consume 

You can open up http://localhost:9080 to get to a Grafana dashboard (no login or dashboards have been setup yet).

Author checklist

• I have manually tested the change and/or added automation in the form of unit tests or integration tests
• I have updated the dependencies list
• I have recompiled and included new protocol buffers to reflect changes I made
• I have added new test fixtures as needed to support added tests
• Check this box if a reviewer can merge this pull request after approval (leave it unchecked if you want to do it yourself)
• I have moved the associated Shortcut story to "Ready for Review"

Reviewer(s) checklist

• I've tried to generate events with several different processes
• I've consumed events from one or more processes
• I've observed the buffer limit putting backpressure on the publishers

[shortcut-integration]

This pull request has been linked to Shortcut Story #9272: Simple Pub/Sub Buffer.

[codecov]

Codecov Report

Merging #7 (6e257b7) into main (5c867af) will decrease coverage by 7.20%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main       #7      +/-   ##
==========================================
- Coverage   67.58%   60.38%   -7.21%     
==========================================
  Files          13       14       +1     
  Lines         725      833     +108     
==========================================
+ Hits          490      503      +13     
- Misses        203      294      +91     
- Partials       32       36       +4     

Flag	Coverage Δ
unittests	60.38% <ø> (-7.21%)	⬇️

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

Impacted Files	Coverage Δ
...ithub.com/rotationalio/ensign/pkg/ensign/server.go	41.02% <0.00%> (-3.27%)	⬇️
...ub.com/rotationalio/ensign/pkg/ensign/o11y/o11y.go	79.12% <0.00%> (-2.70%)	⬇️
...ithub.com/rotationalio/ensign/pkg/ensign/events.go	0.00% <0.00%> (ø)
...ithub.com/rotationalio/ensign/pkg/ensign/pubsub.go	31.57% <0.00%> (ø)

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