Adding diagnostics channels to Fetch

docs/docs/api/DiagnosticsChannel.md

@@ -202,3 +202,60 @@ diagnosticsChannel.channel('undici:websocket:pong').subscribe(({ payload }) => {
   console.log(payload)
 })
 ```
+The below channels collectively act as [`tracingChannel.tracePromise`](https://nodejs.org/api/diagnostics_channel.html#tracingchanneltracepromisefn-context-thisarg-args) on `fetch`. So all of them will publish the arguments passed to `fetch`.
+
+## `tracing:undici:fetch:start`
+
+This message is published when `fetch` is called, and will publish the arguments passed to `fetch`.
+
+```js
+import diagnosticsChannel from 'diagnostics_channel'
+diagnosticsChannel.channel('tracing:undici:fetch:start').subscribe(({ req, input, init, }) => {
+  console.log('input', input)
+  console.log('init', init)
+})
+```
+
+## `tracing:undici:fetch:end`
+
+This message is published at the end of `fetch`'s execution, and will publish any `error` from the synchronous part of `fetch`. Since `fetch` is asynchronous, this should be empty. This channel will publish the same values as `undici:fetch:start`, but we are including it to track when `fetch` finishes execution and to be consistent with [`TracingChannel`](https://nodejs.org/api/diagnostics_channel.html#class-tracingchannel).
+```js
+import diagnosticsChannel from 'diagnostics_channel'
+diagnosticsChannel.channel('tracing:undici:fetch:end').subscribe(({ req, input, init, error }) => {
+  console.log('input', input)
+  console.log('init', init)
+  console.log('error', error) // should be empty
+})
+```
+## `tracing:undici:fetch:asyncStart`
+This message is published after `fetch` resolves or rejects. If `fetch` resolves, it publishes the response in `result`. If it rejects, it publishes the error in `error`.
+```js
+import diagnosticsChannel from 'diagnostics_channel'
+diagnosticsChannel.channel('tracing:undici:fetch:asyncStart').subscribe(({ req, input, init, result, error }) => {
+  console.log('input', input)
+  console.log('init', init)
+  console.log('response', result)
+  console.log('error', error)
+})
+```
+## `tracing:undici:fetch:asyncEnd`
+This channel gets published the same values as and at the same time as `tracing:undici:fetch:asyncStart` in the case of [`tracingChannel.tracePromise`](https://nodejs.org/api/diagnostics_channel.html#tracingchanneltracepromisefn-context-thisarg-args)
+```js
+import diagnosticsChannel from 'diagnostics_channel'
+diagnosticsChannel.channel('tracing:undici:fetch:asyncEnd').subscribe(({ req, input, init, result, error }) => {
+  console.log('input', input)
+  console.log('init', init)
+  console.log('response', result)
+  console.log('error', error)
+})
+```
+## `tracing:undici:fetch:error`
+This message is published when an error is thrown or promise rejects while calling `fetch`.
+```js
+import diagnosticsChannel from 'diagnostics_channel'
+diagnosticsChannel.channel('tracing:undici:fetch:error').subscribe(({ req, input, init, error }) => {
+  console.log('input', input)
+  console.log('init', init)
+  console.log('error', error)
+})
+```

lib/core/diagnostics.js

@@ -6,6 +6,12 @@ const undiciDebugLog = util.debuglog('undici')
 const fetchDebuglog = util.debuglog('fetch')
 const websocketDebuglog = util.debuglog('websocket')
 let isClientSet = false
+let tracingChannel
+
+if (diagnosticsChannel.tracingChannel) {
+  tracingChannel = diagnosticsChannel.tracingChannel('undici:fetch')
+}
+
 const channels = {
   // Client
   beforeConnect: diagnosticsChannel.channel('undici:client:beforeConnect'),
@@ -23,7 +29,9 @@ const channels = {
   close: diagnosticsChannel.channel('undici:websocket:close'),
   socketError: diagnosticsChannel.channel('undici:websocket:socket_error'),
   ping: diagnosticsChannel.channel('undici:websocket:ping'),
-  pong: diagnosticsChannel.channel('undici:websocket:pong')
+  pong: diagnosticsChannel.channel('undici:websocket:pong'),
+  // Fetch channels
+  tracingChannel
 }
 
 if (undiciDebugLog.enabled || fetchDebuglog.enabled) {
@@ -114,6 +122,75 @@ if (undiciDebugLog.enabled || fetchDebuglog.enabled) {
   isClientSet = true
 }
 
+// Track fetch requests
+if (fetchDebuglog.enabled && diagnosticsChannel.tracingChannel) {
+  const debuglog = fetchDebuglog
+
+  tracingChannel.start.subscribe(evt => {
+    const {
+      input
+    } = evt
+    debuglog(
+      'fetch has started request to %s',
+      input
+    )
+  })
+
+  tracingChannel.end.subscribe(evt => {
+    const {
+      input
+    } = evt
+    debuglog(
+      'fetch has received response from %s',
+      input
+    )
+  })
+
+  tracingChannel.asyncStart.subscribe(evt => {
+    const {
+      input,
+      result,
+      error
+    } = evt
+    if (result && error) {
+      debuglog(
+        'fetch has received response for %s - HTTP %d, error is %s',
+        input,
+        result.status,
+        error.message
+      )
+    } else if (result) {
+      debuglog(
+        'fetch has received response for %s - HTTP %d',
+        input,
+        result.status
+      )
+    } else if (error) {
+      debuglog(
+        'fetch has errored for %s - %s',
+        input,
+        error.message
+      )
+    } else {
+      debuglog(
+        'fetch has started request to %s',
+        input
+      )
+    }
+  })
+
+  tracingChannel.error.subscribe(evt => {
+    const {
+      error
+    } = evt
+    debuglog(
+      'fetch error event received response %s',
+      error.message
+    )
+  })
+  isClientSet = true
+}
+
 if (websocketDebuglog.enabled) {
   if (!isClientSet) {
     const debuglog = undiciDebugLog.enabled ? undiciDebugLog : websocketDebuglog