internal/postgres: unexport SearchSource and SearchLatency
internal/postgres: unexport SearchSource and SearchLatency

These variables are only used inside internal/postgres. They are now
unexported and prefixed with "key" for consistency with other tags.

Change-Id: I217616c4cdbe68636ffbff1fb73242aba7ec791a
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/769801
CI-Result: Cloud Build <ea5729d4cf3392b9d382b6ca272c65aa27554f44@system.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite