fix: zombie process on metrics server fail

[toddbaert]

Fixes an issue where a failure of the metrics server to start can cause a zombie process.

This is hard and relatively low value to test with unit tests, but I was able to easily reproduce it with this simple script:

#!/bin/bash

cd flagd && go build -o flagd .

# --port and --sync-port both set to 8015; flagd should exit due to the conflict
./flagd start --port 8015 --sync-port 8015 --uri file:../config/samples/example_flags.flagd.json &
PID=$!
sleep 5

if ps -p $PID > /dev/null 2>&1; then
    echo "OH NO, BUG! Process is still alive despite port conflict."
    kill -9 $PID
    exit 1
fi

echo "NO BUG"

After the simple context handling, the script confirms the bug is fixed.

Fixes: #1807

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[toddbaert]

The mutex+nil-check in the old shutdown goroutines was needed because those goroutines ran concurrently with server setup; the server field might not have been assigned yet when ctx.Done() fired. In the new code, serveWithShutdown is only called after the server is already assigned and receives it as a direct argument, so the race doesn't exist.

[toddbaert]

I extracted this because we use it in a couple places to handle server shutdowns.

[toddbaert]

/gemini review

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[gemini-code-assist]

Code Review

This pull request refactors the server lifecycle management in ConnectService by introducing a serveWithShutdown helper function, which simplifies the Serve method and centralizes graceful shutdown logic. Feedback was provided regarding a potential race condition in serveWithShutdown where a server error could be masked if the context is cancelled at the same time, suggesting a check for errors after the shutdown process completes.