Add OTel Hook

[toddbaert]

Now that OTel 2529 is merged, we should implement the Open Telemetry hook (currently exists as a stub).

Basically, this would entail creating a hook which:

• in the after stage: gets the current span and adds an appropriately named event (feature_flag), with the attributes (feature_flag.key, feature_flag.provider_name, and feature_flag.variant)
• in the error stage, records the exception

In pseudocode, this looks something like:

class OTelHook implements Hook {
  after(context, details) {
    var currentTrace = trace.getActiveSpan();
    currentTrace.addEvent("feature_flag", { "feature_flag.key": details.key, /*  other props */ } );
  }

  error(err) {
    var currentTrace = trace.getActiveSpan();
    currentTrace.recordException(err);
  }
}

See: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/semantic_conventions/feature-flags.md

[bacherfl]

@toddbaert I would like to tackle this issue :)

[toddbaert]

@bacherfl thanks! assigned.