Handle corner cases so that metrics are always updated

[nicmunroe]

No description provided.

[nicmunroe]

This handles two corner cases:

1. The caller's connection drops (or the channel otherwise closes/dies) before any response has been sent.
2. The caller's connection drops (or the channel otherwise closes/dies) after the response has started sending, but before it has completed sending.

Very similar, but different codepaths. In either case now metrics will be updated.

The other big fix was to proper decrement inflight requests and increment processed requests metrics when either of the two above situations occur. In particular this keeps the inflight requests counter from incorrectly growing over time when the corner cases happen.

[codecov-io]

Codecov Report

Merging #89 into master will increase coverage by 0.1%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##             master      #89     +/-   ##
===========================================
+ Coverage     91.15%   91.26%   +0.1%     
- Complexity     2079     2084      +5     
===========================================
  Files           148      148             
  Lines          6029     6036      +7     
  Branches        790      790             
===========================================
+ Hits           5496     5509     +13     
+ Misses          357      355      -2     
+ Partials        176      172      -4

Impacted Files	Coverage Δ	Complexity Δ
...erver/handler/ChannelPipelineFinalizerHandler.java	100% <100%> (ø)	43 <6> (+1)	⬆️
...oste/metrics/codahale/CodahaleMetricsListener.java	100% <100%> (ø)	40 <0> (+1)	⬆️
...ient/asynchttp/netty/StreamingAsyncHttpClient.java	75.86% <0%> (+1.14%)	45% <0%> (+2%)	⬆️
...mpipeline/DownstreamIdleChannelTimeoutHandler.java	90% <0%> (+5%)	5% <0%> (+1%)	⬆️

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