Add MetricsEventSource

[noahfalk]

The feature is still a work in progress but wanted to let others
see it in its current state while I am refining it.

Our out-of-process tools like dotnet-counters and dotnet-monitor need
access to the metrics produced by the new Meter APIs without
requiring the app to take any dependency on a separate OpenTelemetry
library. System.Diagnostics.Metrics EventSource is a new source designed
to let those tools access this data. The EventSource includes high
performance in-proc pre-aggregation capable of observing
millions of instrument invocations/sec/thread with low CPU overhead.

This change does not create any new BCL API surface, the aggregated
data is solely exposed by subscribing to the EventSource such as
using ETW, EventPipe, Lttng, or EventListener. For anyone wanting
in-process APIs to consume the data they could either use MeterListener
for unaggregated data or a library such as OpenTelemetry for
pre-aggregated data.

Todo list:

• - I think we need a configuration on the EventSource to limit the max number of metrics to track. This ensures that even if someone goes nuts with tags we won't accidentally OOM the app trying to insert all those aggregations into the dictionaries.