MetricsListener : Fix exception on shutdown.

When we close the HttpListener that backs the MetricsListener, the
blocking call to GetContext is interrupted and throws an exception.
Adding a catch block to block it and not let it propagate through.
MetricsListener : Remove a case of tight coupling.

Removes the tight coupling between the metrics listener and the
metrics it exposes.
I introduced a JsonReportFormatter, that does exactly that, in the
middle, so that it's possible to provide the listener with a
different set of metrics to expose.
HumanReadableReportFormatter : Makes the GetSample() method virtual.

So that people can override it and customize the source their metrics
come from.
Cleanup : Remove .nupkg files.

We need to build those from the source code, not commit them.
Version : Bump version number to 1.0.9.