[BP of MB-47666] Provide configurable option to disbale logging numDocsPending stat for projector

Projector logs numDocsPending stats every 1min. To that extent, is spawns the go-routines to fetch seqnos from KV node. One go-routine queries ns_server every 10sec to validate the presence of a bucket in the cluster and clear the book-keeping if the bucket wa to be deleted.

In cases where there are large number of buckets, periodically querying all the bucket endpoints is adding significant load on ns_server i.e. in a 30 bucket setup, it would be 30*6 = 180 requests per min. These requests would be to "pools/default/buckets/<bucket_name>?uuid=<>" endpoint which is a relatively costly call.

The goal of this ticket is to provide a configurable option to disable the logging of num_docs_pending stat. Logging this stat is disabled by default and it can be enabled only if some real need for it arises