Search backend: add `Tags()` method to `RepoOpts`

Created by: camdencheek

This adds a Tags() method to repo opts and takes advantage of the new trace.Scoped to make the emitted fields more readable. Additionally, I'd like to start using this information for serializing with our pretty printers, and more granular fields works better for that.

Stacked on https://github.com/sourcegraph/sourcegraph/pull/38349

Test plan

Manually tested that the generated trace tags are readable and correct.

Before:

After: