Add search severity metrics

Why is this change being made?

Search has many different ways of recalling data, as well as data insertion (to be recalled later). As such, the metrics on how "fast" any given operation should be is not straightforward. This MR attempts to categorize and determine the search time performance thresholds we (the Global Search team) are setting for ourselves.

There are several open questions in this MR, lines enclosed in brackets(). E.g. should different SaaS paid tiers expect different search speeds?

Closes gitlab-org/gitlab#258213 (closed)

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to your manager.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.