sparse vectors may have arbitrary number of non zeros and it is hard to
optimize without knowing the actual distribution of nnz. this PR adds a
metric for analyzing that.
issue: https://github.com/milvus-io/milvus/issues/35853
comparing with https://github.com/milvus-io/milvus/pull/38328, this
includes also metric for FTS in query node delegator
also fixed a bug of sparse when searching by pk
Signed-off-by: Buqian Zheng <zhengbuqian@gmail.com>