Add graph in sidekiq for global search indexing queue

This introduces a new graph in the sidekiq dashboard for the incremental index queue size for Global Search (Elasticsearch) indexing.

This metric was introduced in gitlab-org/gitlab!27384 (merged).

An example of the graph with local testing looks like:

Which sits among the other sidekiq queue graphs:

Snapshot example can be found at https://dashboards.gitlab.net/dashboard/snapshot/o7DoiYCWxB6XNzBVHfXH10lyPBa0cioa?orgId=1