Add prom scraper endpoint documentation

Co-authored-by: Merric de Launey <mdelauney@pivotal.io>
Co-authored-by: Rebecca Roberts <robertsre@vmware.com>

Author: MerricdeLauney

docs/internal/README.md

@@ -127,4 +127,9 @@ We do not recommend using internal API endpoints for anything other than their i
 
 **Auth Mechanism:** MTLS + Basic Auth
 
+### POST /internal/v4/metrics
+**Description:** Returns CAPI metrics
 
+**Intended Consumer:** Prom Scraper
+
+**Auth Mechanism:** MTLS