[Bug] concurrent map writes in trackFeatureUsage

[tsurunoyu]

Summary

Using v0.1.9, we are seeing concurrent map write panics in trackFeatureUsage.

This is happening here:

gb.inner.trackedFeatures[key] = res.Value

The callers of trackFeatureUsage seem to have acquired read locks, but nothing is stopping multiple callers from trying to write to this map at the same time.

Expected Behavior

No panic.

Current Behavior

Under load, fetching feature flags via EvalFeature results in a panic.