Update README with N+ license metrics (#1222)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: defanator

README.md

@@ -48,6 +48,7 @@ NGINX Prometheus exporter makes it possible to monitor NGINX or NGINX Plus using
     - [Stream Connections Limiting](#stream-connections-limiting)
     - [Cache](#cache)
     - [Worker](#worker)
+    - [License](#license)
 - [Troubleshooting](#troubleshooting)
 - [Releases](#releases)
   - [Docker images](#docker-images)
@@ -66,7 +67,7 @@ NGINX Prometheus exporter makes it possible to monitor NGINX or NGINX Plus using
 - [Contacts](#contacts)
 - [Contributing](#contributing)
 - [Support](#support)
-- [License](#license)
+- [License](#license-1)
 
 <!-- END doctoc generated TOC please keep comment here to allow auto update -->
 
@@ -432,6 +433,15 @@ Flags:
 | `nginxplus_worker_http_requests_total`   | Counter | The total number of client requests received                             | `id`, `pid` |
 | `nginxplus_worker_http_requests_current` | Gauge   | The current number of client requests that are currently being processed | `id`, `pid` |
 
+#### [License](https://nginx.org/en/docs/http/ngx_http_api_module.html#def_nginx_license_object)
+
+| Name                                                | Type  | Description                                                                                                      | Labels |
+| --------------------------------------------------- | ----- | ---------------------------------------------------------------------------------------------------------------- | ------ |
+| `nginxplus_license_expiration_timestamp_seconds`   | Gauge | License expiration date (expressed as Unix Epoch Time)                                                          | []     |
+| `nginxplus_license_reporting_fails_count`          | Gauge | Number of failed reporting attempts, reset each time the usage report is successfully sent                      | []     |
+| `nginxplus_license_reporting_grace_period_seconds` | Gauge | Number of seconds before traffic processing is stopped after unsuccessful report attempt                        | []     |
+| `nginxplus_license_reporting_healthy`              | Gauge | Indicates whether the reporting state is still considered healthy despite recent failed attempts                | []     |
+
 Connect to the `/metrics` page of the running exporter to see the complete list of metrics along with their
 descriptions. Note: to see server zones related metrics you must configure [status
 zones](https://nginx.org/en/docs/http/ngx_http_api_module.html#status_zone) and to see upstream related metrics you