feat(metrics): integrate prom-client for native Prometheus support

- Add prom-client ^15.1.3 as production dependency
- Migrate Counter class to use prom-client internally
- Migrate Gauge class to use prom-client internally
- Migrate Histogram class to use prom-client internally
- Update getMetricsOutput() to use prom-client's native metrics()
- Update contentType() to use prom-client's contentType
- Enable collectDefaultMetrics() for Node.js system metrics
- Maintain backward compatibility with existing API

Benefits:
- Native OpenMetrics format export
- Proper histogram bucket handling
- Default Node.js metrics (memory, CPU, event loop, GC)
- Better integration with Grafana/Prometheus ecosystem

package-lock.json

@@ -18,6 +18,7 @@
         "music-metadata": "^11.7.0",
         "p-queue": "^9.0.0",
         "pino": "^9.6",
+        "prom-client": "^15.1.3",
         "protobufjs": "^7.2.4",
         "ws": "^8.13.0"
       },
@@ -3115,6 +3116,15 @@
         "@octokit/openapi-types": "^18.0.0"
       }
     },
+    "node_modules/@opentelemetry/api": {
+      "version": "1.9.0",
+      "resolved": "https://registry.npmjs.org/@opentelemetry/api/-/api-1.9.0.tgz",
+      "integrity": "sha512-3giAOQvZiH5F9bMlMiv8+GSPMeqg0dbaeo58/0SlA9sxSqZhnUtxzX9/2FzyhS9sWQf5S0GJE0AKBrFqjpeYcg==",
+      "license": "Apache-2.0",
+      "engines": {
+        "node": ">=8.0.0"
+      }
+    },
     "node_modules/@pinojs/redact": {
       "version": "0.4.0",
       "resolved": "https://registry.npmjs.org/@pinojs/redact/-/redact-0.4.0.tgz",
@@ -4726,6 +4736,12 @@
         "node": ">=0.6"
       }
     },
+    "node_modules/bintrees": {
+      "version": "1.0.2",
+      "resolved": "https://registry.npmjs.org/bintrees/-/bintrees-1.0.2.tgz",
+      "integrity": "sha512-VOMgTMwjAaUG580SXn3LacVgjurrbMme7ZZNYGSSV7mmtY6QQRh0Eg3pwIcntQ77DErK1L0NxkbetjcoXzVwKw==",
+      "license": "MIT"
+    },
     "node_modules/bl": {
       "version": "5.1.0",
       "resolved": "https://registry.npmjs.org/bl/-/bl-5.1.0.tgz",
@@ -11821,6 +11837,19 @@
       ],
       "license": "MIT"
     },
+    "node_modules/prom-client": {
+      "version": "15.1.3",
+      "resolved": "https://registry.npmjs.org/prom-client/-/prom-client-15.1.3.tgz",
+      "integrity": "sha512-6ZiOBfCywsD4k1BN9IX0uZhF+tJkV8q8llP64G5Hajs4JOeVLPCwpPVcpXy3BwYiUGgyJzsJJQeOIv7+hDSq8g==",
+      "license": "Apache-2.0",
+      "dependencies": {
+        "@opentelemetry/api": "^1.4.0",
+        "tdigest": "^0.1.1"
+      },
+      "engines": {
+        "node": "^16 || ^18 || >=20"
+      }
+    },
     "node_modules/promise.allsettled": {
       "version": "1.0.6",
       "resolved": "https://registry.npmjs.org/promise.allsettled/-/promise.allsettled-1.0.6.tgz",
@@ -13651,6 +13680,15 @@
         "url": "https://opencollective.com/synckit"
       }
     },
+    "node_modules/tdigest": {
+      "version": "0.1.2",
+      "resolved": "https://registry.npmjs.org/tdigest/-/tdigest-0.1.2.tgz",
+      "integrity": "sha512-+G0LLgjjo9BZX2MfdvPfH+MKLCrxlXSYec5DaPYP1fe6Iyhf0/fSmJ0bFiZ1F8BT6cGXl2LpltQptzjXKWEkKA==",
+      "license": "MIT",
+      "dependencies": {
+        "bintrees": "1.0.2"
+      }
+    },
     "node_modules/test-exclude": {
       "version": "6.0.0",
       "resolved": "https://registry.npmjs.org/test-exclude/-/test-exclude-6.0.0.tgz",