observability: more clear grouping support

Created by: slimsag

Best reviewed commit-by-commit and with whitespace diffing turned off:

• https://github.com/sourcegraph/sourcegraph/pull/9642/commits/a0b8da87e8b6d0291a21f46addcedd0e60288704?diff=unified&w=1
• https://github.com/sourcegraph/sourcegraph/pull/9642/commits/18faf186b0eaaed94a680f437ecdc54cb2f561e2?diff=unified&w=1
• https://github.com/sourcegraph/sourcegraph/pull/9642/commits/e3599aeb329a0c715429d2eec70e7aaa9b7df221?diff=unified&w=1