FPM health check

Description

The idea is to have a native support for the functionality offered by https://github.com/renatomefi/php-fpm-healthcheck . It means having endpoint that will return selected response (defaulting to 200 without body) if selected metrics in scoreboard are above specified threshold. Those metrics would be configurable in FPM config together with endpoint (listen and path like for status) and the actual response.

The main use case for this functionality are Kubernetes probes and possibly some monitoring tools. It should be more convenient for users and also require less resources then doing periodic status checks which need to collect all metrics.

まず、issue に記載されている PHP-FPM ステータスエンドポイント、scoreboard メトリクス、FPM 設定を確認し、その後 php-fpm-healthcheck の動作と比較します。Kubernetes の probe と監視のユースケースをサポートしながら、設定可能な health エンドポイントで scoreboard のしきい値、listen 設定と path 設定、およびレスポンスを選択できれば完了です。