search: add 15 and 20 second histogram buckets to batch/streaming latency metrics

Created by: ggilmore

Honeycomb links: successful search-blitz monorepo queries, unsuccessful queries

Honeycomb seems to indicate that a decent portion of our search-blitz monorepo results are either:

• taking between 10-15 seconds
• hitting our default 20 second timeout: https://github.com/sourcegraph/sourcegraph/blob/526017e7a5f251c79bcb26cf3629da815ea6a331/internal/search/limits.go#L16

There is a gap in our existing histogram buckets between 10 and 30 seconds. Adding 15 and 20 second buckets should help us get better visibility in our Grafana charts for the above two scenarios.