zoekt: expose dashboard for results of indexing jobs

Created by: ggilmore

This change to zoekt-indexserver includes:

• a new drop down filter that lets you select to drill-down into a metrics for a particular Zoekt instance
• renaming the repo_index_state dashboard to better reflect that the output is a a counter, not a histogram
• adding new dashboards for viewing the indexing state results per instance and viewing the indexing queue length per instance.