fix(mu): add error assertion for missing metrics dependency

servers/mu/src/domain/lib/with-timer-metrics.js

@@ -7,6 +7,9 @@ import { swallow } from '../utils.js'
  * need to be referenced as labels passed to the histogram itself. Any additional checks in this way are up to the implementation and are details of the specific implementation.
  */
 export function withTimerMetrics ({ timer, startLabelsFrom = always({}), stopLabelsFrom = always({}), tracesFrom = always({}), logger = console.warn.bind(console) } = {}) {
+  if (!timer || !timer.startTimer || typeof timer.startTimer !== 'function') {
+    throw new Error('Timer must implement a startTimer function')
+  }
   return (func) => (...funcArgs) => {
     const startLabels = startLabelsFrom(...funcArgs)
     const traces = tracesFrom(...funcArgs)

servers/mu/src/domain/lib/with-timer-metrics.test.js

@@ -3,6 +3,23 @@ import { withTimerMetrics } from './with-timer-metrics.js'
 import assert from 'node:assert'
 
 describe('withTimerMetrics', () => {
+  test('should throw an error if the timer dependency is invalid', () => {
+    assert.throws(() => withTimerMetrics({ timer: {} }), {
+      name: 'Error',
+      message: 'Timer must implement a startTimer function'
+    }, 'Timer must implement a startTimer function')
+
+    assert.throws(() => withTimerMetrics({ timer: { startTimer: 'not a function' } }), {
+      name: 'Error',
+      message: 'Timer must implement a startTimer function'
+    }, 'Timer startTimer property must be a function')
+
+    assert.throws(() => withTimerMetrics({}), {
+      name: 'Error',
+      message: 'Timer must implement a startTimer function'
+    }, 'Timer dependency is required')
+  })
+
   test('should return the original function result', async () => {
     const metricsFunction = withTimerMetrics({
       timer: {