Add AddMetricsConsumer helper to TelemetryConsumption

[MihaZupan]

I am writing networking telemetry docs and it bothered me that we have very convenient helpers for ITelemetryConsumer*s, but nothing for IMetricsConsumer<T>.

So while you can write code like this for events:

services.AddTelemetryConsumer<MyTelemetryConsumer>();

you have to write this for metrics:

services.AddSingleton<IMetricsConsumer<SocketsMetrics>, MetricsConsumer>();
services.AddTelemetryListeners();

or even this if you implement multiple interfaces:

services.AddSingleton<MetricsConsumer>();
services.AddSingleton(services => (IMetricsConsumer<ForwarderMetrics>)services.GetRequiredService<MetricsConsumer>());
services.AddSingleton(services => (IMetricsConsumer<KestrelMetrics>)services.GetRequiredService<MetricsConsumer>());
services.AddSingleton(services => (IMetricsConsumer<SocketsMetrics>)services.GetRequiredService<MetricsConsumer>());
services.AddTelemetryListeners();

This PR adds AddMetricsConsumer helpers that do the same for metrics as AddTelemetryConsumer does for events.

[Tratcher]

?

[MihaZupan]

This doesn't change anything currently, but it can make a significant difference if we ever add debug/trace events to the existing EventSources.
Since this library only knows about the current (informational) events, it would only waste CPU on more verbose events.

If we ever want to consume more verbose events with this library, we will adjust this flag as needed.