Add IsMetricAlreadyCaptured to IMetricsProvider

[gregkalapos]

Solves #729

Prior to GC metrics, all metrics were captured "on the fly", meaning the agent periodically collected metrics (let's say every 30 seconds) and metrics providers were queried for data when the agent wanted to collect the data.

But this does not work in all cases. For collecting GC metrics we subscribe to EventSource (or ETW on Full Framework + Windows) and the GC triggers a method when a GC happens and that is our chance to collect data. GC is just 1 example, I expect similar situations later with other metrics.

It can happen that no GC is triggered for let's say 5 minutes - each time the agent tries to collect data before the 1. GC, it won't see any data.

The MetricsCollector wrongly disabled collecting metrics in such cases - so GC metrics were disabled forever for the given process.

Therefore a bool property IsMetricAlreadyCaptured is added to IMetricsProvider. Until this property does not return true, MetricsCollector won't try to get data from a metrics provider and won't disable it either - it just skips it. The new IsMetricAlreadyCaptured should be set to true when the IMetricsProvider tried to query data or was triggered by e.g. ETW or EventSource the 1. time.