feat(core): Allow adding measurements without global client (#13612)

While working on updating [`sentry-javascript-bundler-plugins` to use v8
of the JavaScript
SDK](getsentry/sentry-javascript-bundler-plugins#579),
I found that I was unable to set measurements as the global client is
not used.

If you're not using a global client, there is currently no way to add
measurements because `Sentry.setMeasurement()` relies on
`getActiveSpan()` which in turn relies on `getCurrentScope()`.

This PR moves the `activeSpan` into the last parameter which defaults to
`getActiveSpan()`.

packages/core/src/tracing/measurement.ts

@@ -6,10 +6,10 @@ import {
 import { getActiveSpan, getRootSpan } from '../utils/spanUtils';
 
 /**
- * Adds a measurement to the current active transaction.
+ * Adds a measurement to the active transaction on the current global scope. You can optionally pass in a different span
+ * as the 4th parameter.
  */
-export function setMeasurement(name: string, value: number, unit: MeasurementUnit): void {
-  const activeSpan = getActiveSpan();
+export function setMeasurement(name: string, value: number, unit: MeasurementUnit, activeSpan = getActiveSpan()): void {
   const rootSpan = activeSpan && getRootSpan(activeSpan);
 
   if (rootSpan) {