[proxy] refactor metrics and add usage logs (#5533)

* **New Features**
  * Access logs now include bytes_upload and bytes_download (API and schemas updated, fields required).
  * Certificate issuance duration is now recorded as a metric.

* **Refactor**
  * Metrics switched from Prometheus client to OpenTelemetry-backed meters; health endpoint now exposes OpenMetrics via OTLP exporter.

* **Tests**
  * Metric tests updated to use OpenTelemetry Prometheus exporter and MeterProvider.

shared/management/http/api/openapi.yml

@@ -2822,6 +2822,16 @@ components:
           type: string
           description: "City name from geolocation"
           example: "San Francisco"
+        bytes_upload:
+          type: integer
+          format: int64
+          description: "Bytes uploaded (request body size)"
+          example: 1024
+        bytes_download:
+          type: integer
+          format: int64
+          description: "Bytes downloaded (response body size)"
+          example: 8192
       required:
         - id
         - service_id
@@ -2831,6 +2841,8 @@ components:
         - path
         - duration_ms
         - status_code
+        - bytes_upload
+        - bytes_download
     ProxyAccessLogsResponse:
       type: object
       properties: