Fixes dotnet#105845

Previously MetricsEventSource wasn't being created for apps that didn't ever create a Meter. This caused a chicken-and-egg problem for RuntimeMetrics which weren't created until MetricsEventSource started a tracing session. This change ensures that MetricsEventSource will be created on demand if ETW, EventPipe, or EventListener based tooling starts a tracing session. I took some extra effort to create the EventSource in a deferred fashion to avoid eager loading System.Diagnostics.DiagnosticSource.dll when it might never be needed.

Aside from the fix there were some small improvements:
- Moved NativeRuntimeEventSource to initialize in the same place as other startup EventSources
- Removed a useless lock(EventListener.EventListenersLock) around EventPipe eventProvider registration