feat(core): Add beforeSendTransaction (#6121)

This adds a new `beforeSendTransaction` option to the SDK. As one would expect from the name, it works just like `beforeSend` (in other words, it's a guaranteed-to-be-last event processor), except that it acts upon transaction events, which the original `beforeSend` doesn't.

Docs PR will be linked in the PR once it's done.

Ref: getsentry/rfcs#19 (comment)

packages/core/src/baseclient.ts

@@ -613,13 +613,17 @@ export abstract class BaseClient<O extends ClientOptions> implements Client<O> {
    * @returns A SyncPromise that resolves with the event or rejects in case event was/will not be send.
    */
   protected _processEvent(event: Event, hint: EventHint, scope?: Scope): PromiseLike<Event> {
-    const { beforeSend, sampleRate } = this.getOptions();
+    const options = this.getOptions();
+    const { sampleRate } = options;
 
     if (!this._isEnabled()) {
       return rejectedSyncPromise(new SentryError('SDK not enabled, will not capture event.', 'log'));
     }
 
     const isTransaction = event.type === 'transaction';
+    const beforeSendProcessorName = isTransaction ? 'beforeSendTransaction' : 'beforeSend';
+    const beforeSendProcessor = options[beforeSendProcessorName];
+
     // 1.0 === 100% events are sent
     // 0.0 === 0% events are sent
     // Sampling for transaction happens somewhere else
@@ -641,17 +645,17 @@ export abstract class BaseClient<O extends ClientOptions> implements Client<O> {
         }
 
         const isInternalException = hint.data && (hint.data as { __sentry__: boolean }).__sentry__ === true;
-        if (isInternalException || isTransaction || !beforeSend) {
+        if (isInternalException || !beforeSendProcessor) {
           return prepared;
         }
 
-        const beforeSendResult = beforeSend(prepared, hint);
-        return _validateBeforeSendResult(beforeSendResult);
+        const beforeSendResult = beforeSendProcessor(prepared, hint);
+        return _validateBeforeSendResult(beforeSendResult, beforeSendProcessorName);
       })
       .then(processedEvent => {
         if (processedEvent === null) {
           this.recordDroppedEvent('before_send', event.type || 'error');
-          throw new SentryError('`beforeSend` returned `null`, will not send event.', 'log');
+          throw new SentryError(`\`${beforeSendProcessorName}\` returned \`null\`, will not send event.`, 'log');
         }
 
         const session = scope && scope.getSession();
@@ -764,12 +768,13 @@ export abstract class BaseClient<O extends ClientOptions> implements Client<O> {
 }
 
 /**
- * Verifies that return value of configured `beforeSend` is of expected type, and returns the value if so.
+ * Verifies that return value of configured `beforeSend` or `beforeSendTransaction` is of expected type, and returns the value if so.
  */
 function _validateBeforeSendResult(
   beforeSendResult: PromiseLike<Event | null> | Event | null,
+  beforeSendProcessorName: 'beforeSend' | 'beforeSendTransaction',
 ): PromiseLike<Event | null> | Event | null {
-  const invalidValueError = '`beforeSend` must return `null` or a valid event.';
+  const invalidValueError = `\`${beforeSendProcessorName}\` must return \`null\` or a valid event.`;
   if (isThenable(beforeSendResult)) {
     return beforeSendResult.then(
       event => {
@@ -779,7 +784,7 @@ function _validateBeforeSendResult(
         return event;
       },
       e => {
-        throw new SentryError(`beforeSend rejected with ${e}`);
+        throw new SentryError(`\`${beforeSendProcessorName}\` rejected with ${e}`);
       },
     );
   } else if (!isPlainObject(beforeSendResult) && beforeSendResult !== null) {

packages/core/src/integration.ts

@@ -65,10 +65,10 @@ export function getIntegrationsToSetup(options: Options): Integration[] {
 
   const finalIntegrations = filterDuplicates(integrations);
 
-  // The `Debug` integration prints copies of the `event` and `hint` which will be passed to `beforeSend`. It therefore
-  // has to run after all other integrations, so that the changes of all event processors will be reflected in the
-  // printed values. For lack of a more elegant way to guarantee that, we therefore locate it and, assuming it exists,
-  // pop it out of its current spot and shove it onto the end of the array.
+  // The `Debug` integration prints copies of the `event` and `hint` which will be passed to `beforeSend` or
+  // `beforeSendTransaction`. It therefore has to run after all other integrations, so that the changes of all event
+  // processors will be reflected in the printed values. For lack of a more elegant way to guarantee that, we therefore
+  // locate it and, assuming it exists, pop it out of its current spot and shove it onto the end of the array.
   const debugIndex = finalIntegrations.findIndex(integration => integration.name === 'Debug');
   if (debugIndex !== -1) {
     const [debugInstance] = finalIntegrations.splice(debugIndex, 1);