Specify time frame in metrics page

What does this MR do?

• Allow linking to a specific start/end time for Metrics page
• Change the time_window query param to start & end

start-end

Known caveats

1. Does not support embed metrics yet (those use default 8 hour window). Some BE changes for that are in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31304

2. Selecting item from the dropdown uses Date.now() so you can only use it to get the current window. If you want a different time range around the incident you need to manually edit timestamps for now

i.e. It is 6PM.
I link someone to metrics from 4:00-4:30PM
The dropdown reads "30 minutes"
I select "Last 3 Hours" on the dropdown
It gives me 3-6pm, not 130-430

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

FE part of #62973 (closed)