There is a new checker field called statsCollector. This contains
a map of stats pointers where the key is a stringified redundancy
scheme. stats contains all tagged monkit metrics. These metrics exist
under the key name, "tagged_repair_stats", which is tagged with the
name of each metric and a corresponding rs scheme.
As the metainfo observer works on a segment, it checks statsCollector
for a stats corresponding to the segment's redundancy scheme. If one
doesn't exist, it is created and chained to the monkit scope. Now we can call
Observe, Inc, etc on the fields just like before, and they have tags!
durabilityStats has also been renamed to aggregateStats.
At the end of the metainfo loop, we insert the aggregateStats totals into the
corresponding stats fields for metric reporting.
Change-Id: I8aa1918351d246a8ef818b9712ed4cb39d1ea9c6
2ac72eaf16