fix(metrics): Update return type of beforeSendMetric (#18261)

Author: chargome

packages/core/src/types-hoist/options.ts

@@ -420,9 +420,9 @@ export interface ClientOptions<TO extends BaseTransportOptions = BaseTransportOp
    * @default undefined
    *
    * @param metric The metric generated by the SDK.
-   * @returns A new metric that will be sent.
+   * @returns A new metric that will be sent | null.
    */
-  beforeSendMetric?: (metric: Metric) => Metric;
+  beforeSendMetric?: (metric: Metric) => Metric | null;
 
   /**
    * Function to compute tracing sample rate dynamically and filter unwanted traces.