Add reproxy metrics metadata to developer reproxy.cfg

Values are as per go/re-client-chromium-developer-metric-collection-op#heading=h.oh0ozwgordj5

Change-Id: Ib5f1fa1843edafdfa817390cbd5ad8fff15a2016
Bug: b/281504726
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4513215


Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Auto-Submit: Ben Segall <bentekkie@google.com>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148344}