feat: serve sync.proto on port 8015

[Kavindu-Dodan]

This PR

Introduce flag sync capability to flagd as discussed at [1] and fixes #1230 .

What's changed ?

The change included with this PR introduces the gRPC sync contract [2]. This allows flagd to expose its store to in-process provider which consumes gRPC flag stream.

How to use ?

flagd will start sync service on port 8015. You can alter the default sync port by providing desired port to the startup flag syncPort (--sync-port=8686)

Implementation details

• Sync service startup is delayed to allow configured flag sources to complete their initial loading (dealy is 5 seconds)
• Sync request's selector can be used to specify the specific flag source. If unset, all flags will be sent
• A new stream response will be created whenever there are updates from flag sources

[1] - #1153
[2] - https://github.com/open-feature/flagd-schemas/blob/main/protobuf/flagd/sync/v1/sync.proto

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.11%. Comparing base (1c530ab) to head (ae6a547).
Report is 33 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1237      +/-   ##
==========================================
+ Coverage   73.69%   77.11%   +3.42%     
==========================================
  Files          32       19      -13     
  Lines        3140     1547    -1593     
==========================================
- Hits         2314     1193    -1121     
+ Misses        717      273     -444     
+ Partials      109       81      -28     

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[toddbaert]

When starting flagd like this:

go run main.go start -f file:../config/samples/example_flags.flagd.json -e

Then attempting to resolve JUST that selector like this:

grpcurl -import-path "{path-to-flagd-repo}/schemas/protobuf/flagd/sync/v1/" -proto sync.proto  -plaintext -d '{"selector":"file:../config/samples/example_flags.flagd.json"}' localhost:8015  flagd.sync.v1.FlagSyncService/FetchAllFlags

I get:

ERROR:
  Code: Unknown
  Message: no flag watcher setup for source file:../config/samples/example_flags.flagd.json

Is there some normalization happening? I would have thought I could select based on this.

Note that omitting the -d {} arg (selecting nothing) works fine and returns all the flags as expected.

EDIT:

-d '{"selector":"../config/samples/example_flags.flagd.json"}' works! (your comment above led me to this solution).

This seems a bit brittle and confusing, but I think it's out of scope of this PR. I think we should just document it for now, and perhaps add a new PR that allows users to define an friendly label for each source in the JSON config mode.

[Kavindu-Dodan]

Merging with all the approvals. Thank you @thisthat @toddbaert @bacherfl @beeme1mr for reviews 🤝