fix: force to string to satisfy loki api

src/log_builder.ts

@@ -78,6 +78,28 @@ export class LogBuilder {
     return labels
   }
 
+  /**
+   * Loki structured metadata requires string values only.
+   */
+  #buildStructuredMetadata(
+    log: PinoLog,
+    structuredMetaKey?: string,
+  ): Record<string, string> | undefined {
+    if (!structuredMetaKey) return undefined
+
+    const meta = log[structuredMetaKey]
+    if (!meta || typeof meta !== 'object') return undefined
+
+    const result: Record<string, string> = {}
+    for (const [key, value] of Object.entries(meta)) {
+      if (typeof value === 'string') result[key] = value
+      else if (typeof value === 'object' && value !== null) result[key] = JSON.stringify(value)
+      else result[key] = String(value)
+    }
+
+    return result
+  }
+
   /**
    * Convert a level to a human readable status
    */
@@ -103,9 +125,7 @@ export class LogBuilder {
     const hostname = options.log.hostname
     options.log.hostname = undefined
 
-    const structuredMetadata: Record<string, any> = options.structuredMetaKey
-      ? options.log[options.structuredMetaKey]
-      : undefined
+    const structuredMetadata = this.#buildStructuredMetadata(options.log, options.structuredMetaKey)
 
     const formattedMessage = options.logFormat
       ? formatLog({

tests/unit/log_builder.spec.ts

@@ -182,6 +182,34 @@ test.group('Log Builder', () => {
     assert.deepEqual(log.values[0][2], { foo: 'bar' })
   })
 
+  test('structured metadata converts non-string values to strings', ({ assert }) => {
+    const logBuilder = new LogBuilder()
+
+    const log = logBuilder.build({
+      log: {
+        level: 30,
+        msg: 'hello world',
+        metaKey: {
+          string: 'bar',
+          number: 200,
+          boolean: true,
+          nested: { deep: 'value' },
+          array: [1, 2, 3],
+        },
+      },
+      replaceTimestamp: true,
+      structuredMetaKey: 'metaKey',
+    })
+
+    assert.deepEqual(log.values[0][2], {
+      string: 'bar',
+      number: '200',
+      boolean: 'true',
+      nested: '{"deep":"value"}',
+      array: '[1,2,3]',
+    })
+  })
+
   test('does not include structured metadata when not set', ({ assert }) => {
     const logBuilder = new LogBuilder()