Update Metrics Exporter README

The updated readme removes text indicating that the exporter is still in
alpha.

README.md

@@ -36,8 +36,6 @@ See [Tracing Readme](exporters/trace/README.md) for installation and usage instr
 
 ## Google Cloud Monitoring Exporter
 
-*Note: This is an alpha-release.*
-
 See [Metrics Readme](exporters/metrics/README.md) for installation and usage instructions.
 
 ## Google Cloud Autoconfigure module

exporters/metrics/README.md

@@ -2,8 +2,6 @@
 
 [![Maven Central][maven-image]][maven-url]
 
-*NOTE: While Opentelemetry Metrics API is stable, the SDK is still Alpha in OpenTelemetry, and this exporter is not guaranteed to work across versions.*
-
 Opentelemetry Google Monitoring Metrics Exporter allows users to send collected metrics
 to Google Cloud.
 
@@ -15,13 +13,23 @@ Google Cloud Monitoring is a managed service provided by Google Cloud Platform.
 
 ## Usage
 
-TODO(jsuereth): Write this.
-
 See [the code example](../../examples/metrics) for details.
 
 ## Authentication
 
-TODO(jsuereth): Write this section.
+If you are running in a GCP environment, the exporter will automatically authenticate using the environment's service account. If not, you will need to follow the instructions in Authentication.
+
+This exporter uses [google-cloud-java](https://github.com/googleapis/google-cloud-java), which uses the [google-auth-library-java](https://github.com/googleapis/google-auth-library-java) for authentication. For details about how to configure the authentication see [here](https://github.com/googleapis/google-cloud-java#authentication).
+The credentials can be passed into the exporter via the `setCredentials` method in the [MetricConfiguration](https://github.com/GoogleCloudPlatform/opentelemetry-operations-java/blob/7a8dcf420f5551bf0df969f83928b18c443da5aa/exporters/metrics/src/main/java/com/google/cloud/opentelemetry/metric/MetricConfiguration.java).
+
+If you prefer to manually set the credentials use:
+```java
+MetricConfiguration.builder()
+    .setCredentials(new GoogleCredentials(new AccessToken(accessToken, expirationTime)))
+    .setProjectId("MyProjectId")
+    .build();
+```
+before passing it into the MetricExporter constructor.
 
 ## Configuration