BUG: `MultiWriter.DefineMetrics` always returns `nil`, silently discarding errors from child writers

[Mahmoud-Ayman-Saleh]

MultiWriter.DefineMetrics correctly accumulates errors from child writers using errors.Join, but then unconditionally returns nil instead of the accumulated error, silently discarding all failures

[0xgouda]

I guess it's not a bug, prom's DefineMetrics() always returns nil anyways, and for the grpc sink iirc we left it that way for now because its DefineMetrics() method is still in an experimental stage and will potentially cause a flood of errors in the logs if the corresponding server doesn't implement it.