Backend: add scoped `log.Field` encoder
Created by: camdencheek
This adds the function trace.Scoped()
to the utility functions for trace fields. This returns a lazy logger that adds a scope to each of the fields so they are emitted with the key scope.originalKey
rather than just originalKey
. The purpose of this is so I can nest logical sub-fields in traces.
Test plan
Manually tested as part of https://github.com/sourcegraph/sourcegraph/pull/38350