docs: clean up BetaApi annotations from built in metrics API (#1741)

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:
- [ ] Make sure to open an issue as a [bug/issue](https://togithub.com/googleapis/java-bigtable/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
- [ ] Ensure the tests and linter pass
- [ ] Code coverage does not decrease (if any source code was changed)
- [ ] Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> ☕️

If you write sample code, please follow the [samples format](
https://togithub.com/GoogleCloudPlatform/java-docs-samples/blob/main/SAMPLE_FORMAT.md).

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/BigtableDataSettings.java

@@ -197,13 +197,7 @@ public static void enableGfeOpenCensusStats() {
     com.google.cloud.bigtable.data.v2.stub.metrics.RpcViews.registerBigtableClientGfeViews();
   }
 
-  /**
-   * Register built in metrics.
-   *
-   * <p>This is an experimental feature. Please fill up this form to have your project allow listed
-   * for the private preview: https://forms.gle/xuhu6vCunn2MjV2m9
-   */
-  @BetaApi("Built in metric is not currently stable and may change in the future")
+  /** Register built in metrics. */
   public static void enableBuiltinMetrics() throws IOException {
     if (BUILTIN_METRICS_REGISTERED.compareAndSet(false, true)) {
       BuiltinViews.registerBigtableBuiltinViews();
@@ -214,11 +208,7 @@ public static void enableBuiltinMetrics() throws IOException {
   /**
    * Register built in metrics with credentials. The credentials need to have metric write access
    * for all the projects you're publishing to.
-   *
-   * <p>This is an experimental feature. Please fill up this form to have your project allow listed
-   * for the private preview: https://forms.gle/xuhu6vCunn2MjV2m9
    */
-  @BetaApi("Built in metric is not currently stable and may change in the future")
   public static void enableBuiltinMetrics(Credentials credentials) throws IOException {
     if (BUILTIN_METRICS_REGISTERED.compareAndSet(false, true)) {
       BuiltinViews.registerBigtableBuiltinViews();