Implement filtering of async requests.

[lapo-luchini]

This fixes #676 for me… but there are no tests as I have no idea how to properly test an async servlet responses with Mockito.

Doesn't break any of the existing tests, though:

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 T E S T S
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Running io.prometheus.client.exporter.MetricsServletTest
Tests run: 4, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.548 sec - in io.prometheus.client.
exporter.MetricsServletTest
Running io.prometheus.client.filter.MetricsFilterTest
Tests run: 7, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.232 sec - in io.prometheus.client.
filter.MetricsFilterTest

Results :

Tests run: 11, Failures: 0, Errors: 0, Skipped: 0