Improve how metrics are captured from the OTel receiver

[yvrhdn]

Is your feature request related to a problem? Please describe.

The distributor embeds the OTel receiver to receive traces. We currently very selectively export the metrics and convert them to Prometheus metrics. This means not all metrics are exposed and we might miss information.

Describe the solution you'd like

Export all metrics emitted by the receiver code and push them into our Prometheus registry.

Describe alternatives you've considered

We don't have to do this and keep the current situation.

Additional context

When we create the receiver we pass these TelemetrySettings:

tempo/modules/distributor/receiver/shim.go

Lines 274 to 284 in f00ed6a

	params := receiver.CreateSettings{
	ID: component.NewIDWithName(nopType, fmt.Sprintf("%s_receiver", componentID.Type().String())),
	TelemetrySettings: component.TelemetrySettings{
	Logger: zapLogger,
	TracerProvider: traceProvider,
	MeterProvider: meterProvider,
	ReportStatus: func(*component.StatusEvent) {
	},
	},
	}
	receiver, err := factoryBase.CreateTracesReceiver(ctx, params, cfg, middleware.Wrap(shim))

The custom MeterProvider is implemented here: https://github.com/grafana/tempo/blob/main/modules/distributor/receiver/metrics_provider.go

We could instead create a MeterProvider that exports to our prometheus registry. The code would look something like this:

    // create an OTel metrics -> Prometheus exporter
	exporterOpts := []otelexporterprometheus.Option{
		otelexporterprometheus.WithRegisterer(reg),  // this is our main prometheus.Registry
        // optional, depends if you want otel_scope_info and otel_target_info metrics or not
		otelexporterprometheus.WithoutScopeInfo(),
		otelexporterprometheus.WithoutTargetInfo(),
	}
	exporter, err := otelexporterprometheus.New(exporterOpts...)
	// handle err

    // create a MeterProvider with the exporter as reader
	meterProviderOpts := []sdkmetric.Option{
		sdkmetric.WithReader(exporter),
	}
	meterProvider := sdkmetric.NewMeterProvider(meterProviderOpts...)

Then you can pass this meterProvider to TelemetrySettings.

[yvrhdn]

I took a quick stab at this and using the exporter changes the metric names. If we want to keep the same metrics we will need to setup a view to rename metrics I think. I'm not super familiar with how that works tbh.

As a first step I set up a test to check the metrics are being emitted correctly: #4477

(I probably won't look further into this unless it prevents us from upgrading OTel)