FTS - Prune stats responses

Today the stats endpoint response is very verbose and this could result in more processing and storage needs at the ns_server level.

For eg:

"travel-sample:FTS:avg_grpc_internal_queries_latency": 0,

"travel-sample:FTS:avg_grpc_queries_latency": 0,

"travel-sample:FTS:avg_internal_queries_latency": 0, 

The prefixes like $bucketName:$Scope:$CollectionName:$IndexName:StatsName would become really inefficient. And the overheads would be really huge with more indexes.

This needs to be fixed with a few stats trimming approaches.

Couple of them popping to mind are

1. Hierarchical data/response representation to result in really trimmed version of relevant stats.
2. Adoption of gzip or other compression utilities/custom encodings if the request `Content - Type` demands it.

But any such stats response change would result in an API break and needs to be checked / Synced with ns_server team before proceeds further.