added shard store size as well as "primary" label to shard metrics

[macgyver603]

refs #156

I added shards_store_size_in_bytes as one of the shard-level metrics exposed through the exporter.

I also added a primaries label to the shard-level metrics in order to differentiate between primaries and replicas.

As for counting replica shards, there isn't a specific metric for tracking that, but a quick 'n dirty way of getting that would be to count the number of of occurrences of a specific metric for a specific shard with something like:

count(elasticsearch_indices_shards_docs_deleted{index="my-index", shard="0"})

[zwopir]

thanks for your PR, looks good. Merged