Add a metric for processed keys when rebuilding indexes.

It would be useful to understand how long it will take to rebuild indexes, since there can be a lot of data and indexes. Now there are following metrics that allow to estimate approximately how many indexes are left to rebuild:

1. IsIndexRebuildInProgress - rebuilding cache indexes in the process;
2. IndexBuildCountPartitionsLeft - remaining number of partitions (by cache group) to rebuild indexes for.

For a more accurate estimate, I suggest adding a metric for caches "Number of keys processed when rebuilding indexes" with the name "IndexRebuildKeyProcessed". This way we can estimate for cache how much index rebuilding will take. To do this, we can get "CacheSize" and use new metric to find out how many keys are left to process.

I also suggest adding methods:

1. org.apache.ignite.cache.CacheMetrics#getIndexRebuildKeyProcessed
2. org.apache.ignite.cache.CacheMetrics#IsIndexRebuildInProgress