WebHDFS metrics

[Laxman Ch <la...@gmail.com>]

Hi Devs,

We recently enabled WebHDFS on Hadoop 2.6.0.
I couldn't find any traces of metrics in code/documentation for WebHDFS.
I feel we need to expose some metrics that may help in
debugging/identifying some problems.

*Thoughts on this please?*

Here is a quick compilation of metrics to start with
- Total requests count
- Failed requests count
- Success request count
- Total Request count by type (GET, POST, CREATE, etc.,)
- Failed Request count by type (GET, POST, CREATE, etc.,)
- Success request count by response type (OK - 200, REDIRECT - 307, etc.,)
- Total bytes read
- Total bytes written

I can work on this feature if someone else finds this useful.
-- 
Thanks,
Laxman