Add missing documentation on cert metrics (#21073)

Signed-off-by: Alexander Scheel <[email protected]>

website/content/api-docs/secret/pki.mdx

@@ -3898,6 +3898,21 @@ The below parameters are in addition to the regular parameters accepted by the
   operations; note that this is from the end of one operation to the start of
   the next so the time of the operation itself does not need to be considered.
 
+- `maintain_stored_certificate_counts` `(bool: false)` - When enabled,
+  maintains expensive counts of certificates. During initialization of the
+  mount, a LIST of all certificates is performed to get a baseline figure and
+  throughout operations like issuance, revocation, and subsequent tidies, the
+  figure is updated.
+
+~> *Note*: It is strongly recommend to not enable this value if 50k or more
+   certificates are stored in the mount or if many PKI mounts are in use in
+   this cluster. Instead, use audit logs and aggregate this data externally
+   to Vault so as not to impact Vault performance.
+
+- `publish_stored_certificate_count_metrics` `(bool: false)` - When enabled,
+  publishes the value computed by `maintain_stored_certificate_counts` to
+  the mount's metrics. This requires the former to be enabled.
+
 #### Sample Payload
 
 ```json