add docs for deptracker_post_record_unknown_artifact_cache_hit

Signed-off-by: Eric Pickard <piceri@github.com>

Author: piceri

README.md

@@ -195,7 +195,10 @@ The metrics exposed beyond the default Prometheus metrics are:
   that were rate limited.
 * `deptracker_post_record_unknown_artifact`: the number of attempts
   that resulted in no matching attestation for the container digest
-  (404 "no artifacts found" responses).
+  (404 "no artifacts found" responses) and an entry in the unknown
+  artifact cache.
+* `deptracker_post_record_unknown_artifact_cache_hit`: the number of 
+  attempts to create new records prevented by the unknown artifact cache.
 * `deptracker_post_record_soft_fail`: the number of recoverable failed
   attempts to upload the deployment record.
 * `deptracker_post_record_hard_fail`: the number of failures to