Add docs for new sys/billing/certificates endpoint

Author: rculpepper

content/vault/v1.21.x (rc)/content/api-docs/system/billing-pki-certificate-count.mdx

@@ -0,0 +1,61 @@
+---
+layout: api
+page_title: /sys/billing/certificates - HTTP API
+description: >-
+  The `/sys/billing/certificates` endpoint is used to return data about the number of PKI certificates issued by Vault.
+---
+
+`# /sys/billing/certificates`
+
+The `/sys/billing/certificates` endpoint is used to return data about the number of PKI certificates issued by Vault.
+It returns information for the entire cluster.
+
+## PKI Certificate Counts
+
+This endpoint returns the number of PKI certificates issued by Vault in each month of the given time range.
+If there are no counts for a given month, an error will be returned for that month.
+
+| Method | Path                        |
+| :----- | :-------------------------- |
+| `GET`  | `/sys/billing/certificates` |
+
+### Request Parameters
+
+- `start_time` `(string: <required>)` - The first month for which to return certificate counts.
+The time must be a timestamp in RFC 3339 format (e.g., `2025-01-01T00:00:00Z` for January 2025).
+
+- `end_time` `(string: <required>)` - The last month for which to return certificate counts. The `end_time` must not be before the `start_time`.
+The time must be a timestamp in RFC 3339 format (e.g., `2025-01-01T00:00:00Z` for January 2025).
+
+### Sample Payload
+
+```json
+{
+    "start_time": "2025-01-01T00:00:00Z",
+    "end_time": "2025-01-01T00:00:00Z"
+}
+```
+
+### Sample request
+
+```shell-session
+$ curl \
+    --header "X-Vault-Token: ..." \
+    --data @payload.json \
+    http://127.0.0.1:8200/v1/sys/billing/certificates
+```
+
+### Sample response
+
+```json
+{
+    "months": [
+        {
+            "timestamp": "2025-01-01T00:00:00Z",
+            "counts": {
+                "issued_certificates": 123
+            }
+        }
+    ]
+}
+```