feat(server/metrics): add documentation around the new metrics endpoint

apps/server/src/assets/doc_notes/en/User Guide/User Guide/Advanced Usage/Attributes/Labels.html

@@ -275,9 +275,9 @@
             content via the injected <code>now</code> and <code>parentNote</code> variables.</p>
           <p>Examples:</p>
           <ul>
-            <li><code>${parentNote.getLabel('authorName')}'s literary works</code>
+            <li><code><span class="math-tex">\({parentNote.getLabel('authorName')}'s literary works</span></code>
             </li>
-            <li><code>Log for ${now.format('YYYY-MM-DD HH:mm:ss')}</code>
+            <li><code>Log for \){now.format('YYYY-MM-DD HH:mm:ss')}</code>
             </li>
             <li>to mirror the parent's template.</li>
           </ul>

apps/server/src/assets/doc_notes/en/User Guide/User Guide/Advanced Usage/Metrics.html

@@ -0,0 +1,88 @@
+<p>&nbsp;</p>
+<h1><strong>Trilium Metrics API</strong></h1>
+<p>The Trilium metrics API provides comprehensive monitoring data about your
+  Trilium instance, designed for external monitoring systems like Prometheus.</p>
+<h2><strong>Endpoint</strong></h2>
+<ul>
+  <li><strong>URL</strong>: <code>/etapi/metrics</code>
+  </li>
+  <li><strong>Method</strong>: <code>GET</code>
+  </li>
+  <li><strong>Authentication</strong>: ETAPI token required</li>
+  <li><strong>Default Format</strong>: Prometheus text format</li>
+</ul>
+<h2><strong>Authentication</strong></h2>
+<p>You need an ETAPI token to access the metrics endpoint. Get one by:</p><pre><code class="language-text-x-trilium-auto"># Get an ETAPI token
+curl -X POST http://localhost:8080/etapi/auth/login \
+  -H "Content-Type: application/json" \
+  -d '{"password": "your_password"}'
+</code></pre>
+<h2><strong>Usage</strong></h2>
+<h3><strong>Prometheus Format (Default)</strong></h3><pre><code class="language-text-x-trilium-auto">curl -H "Authorization: YOUR_ETAPI_TOKEN" \
+  http://localhost:8080/etapi/metrics
+</code></pre>
+<p>Returns metrics in Prometheus text format:</p><pre><code class="language-text-x-trilium-auto"># HELP trilium_info Trilium instance information
+# TYPE trilium_info gauge
+trilium_info{version="0.91.6",db_version="231",node_version="v18.17.0"} 1 1701432000
+
+# HELP trilium_notes_total Total number of notes including deleted
+# TYPE trilium_notes_total gauge
+trilium_notes_total 1234 1701432000
+</code></pre>
+<h3><strong>JSON Format</strong></h3><pre><code class="language-text-x-trilium-auto">curl -H "Authorization: YOUR_ETAPI_TOKEN" \
+  "http://localhost:8080/etapi/metrics?format=json"
+</code></pre>
+<p>Returns detailed metrics in JSON format for debugging or custom integrations.</p>
+<h2><strong>Available Metrics</strong></h2>
+<h3><strong>Instance Information</strong></h3>
+<ul>
+  <li><code>trilium_info</code> - Version and build information with labels</li>
+</ul>
+<h3><strong>Database Metrics</strong></h3>
+<ul>
+  <li><code>trilium_notes_total</code> - Total notes (including deleted)</li>
+  <li><code>trilium_notes_deleted</code> - Number of deleted notes</li>
+  <li><code>trilium_notes_active</code> - Number of active notes</li>
+  <li><code>trilium_notes_protected</code> - Number of protected notes</li>
+  <li><code>trilium_attachments_total</code> - Total attachments</li>
+  <li><code>trilium_attachments_active</code> - Active attachments</li>
+  <li><code>trilium_revisions_total</code> - Total note revisions</li>
+  <li><code>trilium_branches_total</code> - Active branches</li>
+  <li><code>trilium_attributes_total</code> - Active attributes</li>
+  <li><code>trilium_blobs_total</code> - Total blob records</li>
+  <li><code>trilium_etapi_tokens_total</code> - Active ETAPI tokens</li>
+  <li><code>trilium_embeddings_total</code> - Note embeddings (if available)</li>
+</ul>
+<h3><strong>Categorized Metrics</strong></h3>
+<ul>
+  <li><code>trilium_notes_by_type{type="text|code|image|file"}</code> - Notes
+    by type</li>
+  <li><code>trilium_attachments_by_type{mime_type="..."}</code> - Attachments
+    by MIME type</li>
+</ul>
+<h3><strong>Statistics</strong></h3>
+<ul>
+  <li><code>trilium_database_size_bytes</code> - Database size in bytes</li>
+  <li><code>trilium_oldest_note_timestamp</code> - Timestamp of oldest note</li>
+  <li><code>trilium_newest_note_timestamp</code> - Timestamp of newest note</li>
+  <li><code>trilium_last_modified_timestamp</code> - Last modification timestamp</li>
+</ul>
+<h2><strong>Prometheus Configuration</strong></h2>
+<p>Add to your <code>prometheus.yml</code>:</p><pre><code class="language-text-x-trilium-auto">scrape_configs:
+  - job_name: 'trilium'
+    static_configs:
+      - targets: ['localhost:8080']
+    metrics_path: '/etapi/metrics'
+    headers:
+      Authorization: 'YOUR_ETAPI_TOKEN'
+    scrape_interval: 30s
+</code></pre>
+<h2><strong>Error Responses</strong></h2>
+<ul>
+  <li><code>400</code> - Invalid format parameter</li>
+  <li><code>401</code> - Missing or invalid ETAPI token</li>
+  <li><code>500</code> - Internal server error</li>
+</ul>
+<p>&nbsp;</p>
+<p>&nbsp;</p>
+<p>&nbsp;</p>