Fix metrics reporting to also report return codes correctly.

[c0d1ngm0nk3y]

The return codes were always reported as NOERROR because the was no handler called inside the metrics handler where the return code could be recorded.

So instead of calling all handlers in serial (metrics, cache, forward), the metrics handler calls the next handler during reporting (metrics[cache, forward]).

This only applies if reporting metrics is enabled.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/174944494

The labels on this github issue will be updated when the story is started.

[beyhan]

@mrosecrance will be nice if we can bring this in. This change doesn't effect the bosh-dns functionality. It has only an effect when metrics are enabled. We want to enable bosh-dns metrics ASAP on our landscapes, because we found them very useful.

[beyhan]

Thanks for looking into this and merging!