search: Add prometheus metric for overall search latency
Created by: camdencheek
To avoid further increasing the cardinality of the
src_graphql_field_seconds metric, this commit creates a new exported
prometheus metric src_search_response_latency_seconds which only
tracks the response latency for a single search query.
Additionally, it adds support for parsing the search-blitz query name
from the User-Agent string as the source so that we can uniquely
identify named queries to use as sentinels of search performance.
Example showing it works in grafana:
