Fix mixin user id format

[aldernero]

What this PR does

Which issue(s) this PR fixes or relates to

This PR fixes 3 issues:

1. The user/tenant id is sometimes abbreviated as a number in the tables in the Top Tenants dashboard. For example, the id 123456 might get abbreviated as 123K, making it not useful for debugging purposes. The auto formatting of the field seems to depend on what else is in the table and possible Grafana version. This PR makes the field format explicitly a string.
2. In transformations the value column used to show up as "Value #A", so that's how it's referenced in the same tables as for issue 1. The column is now just "Value". In the tables the alias for the column no longer shows up, since "Value #A" doesn't match. This PR fixes that so the aliases show up in the column header.
3. Fixes a typo in the Slow Queries dashboard

Checklist

• [N/A] Tests updated.
• [N/A] Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• [N/A] about-versioning.md updated with experimental features.

[pstibrany]

Thank you! I think this bugfix is worthy a note in CHANGELOG (mixin section).