Add 'search_query_latency_mean' and 'search_index_latency_mean' metrics [JIRA: RIAK-1611]

[dmitrizagidulin]

I suspect the search-related _mean metrics got accidentally left out from this:
basho/riak_kv#855

We have all the others (median, 95, etc), but not mean.
Can we add them?

[zeeshanlakhani]

@uwiger wondering if you have any insight into this? Thanks!

[uwiger]

I have no objection. In the equivalence tests, metrics that had been added, that were not there in the folsom version, were flagged as errors. Appropriate at the time, I guess, but moving forward, you should of course decide which metrics you want to add.

[zeeshanlakhani]

@uwiger cool, that's what i figured... just replicating folsom metrics at the time. thanks.