doc(storage): update gRPC docs (googleapis#12031)

Clarify auth and other requirements for direct connectivity. Reduce metrics docs and rely on link to cloud docs instead.

Author: tritone

storage/doc.go

@@ -333,12 +333,13 @@ to add a [custom audit logging] header:
 
 # gRPC API
 
-This package includes support for the Cloud Storage gRPC API. The
-implementation uses gRPC rather than the Default
-JSON & XML APIs to make requests to Cloud Storage.
-The Go Storage gRPC client is generally available.
-The Notifications, Serivce Account HMAC
-and GetServiceAccount RPCs are not supported through the gRPC client.
+This package includes support for the [Cloud Storage gRPC API]. This
+implementation uses gRPC rather than the default JSON & XML APIs
+to make requests to Cloud Storage. All methods on the [Client] support
+the gRPC API, with the exception of [GetServiceAccount], [Notification],
+and [HMACKey] methods.
+
+The Cloud Storage gRPC API is generally available.
 
 To create a client which will use gRPC, use the alternate constructor:
 
@@ -349,43 +350,31 @@ To create a client which will use gRPC, use the alternate constructor:
 	}
 	// Use client as usual.
 
-Using the gRPC API inside GCP with a bucket in the same region can allow for
-[Direct Connectivity] (enabling requests to skip some proxy steps and reducing
-response latency). A warning is emmitted if gRPC is not used within GCP to
-warn that Direct Connectivity could not be initialized. Direct Connectivity
-is not required to access the gRPC API.
+One major advantage of the gRPC API is that it can use [Direct Connectivity],
+enabling requests to skip some proxy steps and reducing responce latency.
+Requirements to use Direct Connectivity include:
+
+  - Your application must be running inside Google Cloud.
+  - Your Cloud Storage [bucket location] must overlap with your VM or compute
+    environment zone. For example, if your VM is in us-east1a, your bucket
+    must be located in either us-east1 (single region), nam4 (dual region),
+    or us (multi-region).
+  - Your client must use service account authentication.
+
+Additional requirements for Direct Connectivity are documented in the
+[Cloud Storage gRPC docs].
 
 Dependencies for the gRPC API may slightly increase the size of binaries for
 applications depending on this package. If you are not using gRPC, you can use
 the build tag `disable_grpc_modules` to opt out of these dependencies and
 reduce the binary size.
 
-The gRPC client emits metrics by default and will export the
-gRPC telemetry discussed in [gRFC/66] and [gRFC/78] to
-[Google Cloud Monitoring]. The metrics are accessible through Cloud Monitoring
-API and you incur no additional cost for publishing the metrics. Google Cloud
-Support can use this information to more quickly diagnose problems related to
-GCS and gRPC.
-Sending this data does not incur any billing charges, and requires minimal
-CPU (a single RPC every minute) or memory (a few KiB to batch the
-telemetry).
-
-To access the metrics you can view them through Cloud Monitoring
-[metric explorer] with the prefix `storage.googleapis.com/client`. Metrics are emitted
-every minute.
-
-You can disable metrics using the following example when creating a new gRPC
-client using [WithDisabledClientMetrics].
-
-The metrics exporter uses Cloud Monitoring API which determines
-project ID and credentials doing the following:
-
-* Project ID is determined using OTel Resource Detector for the environment
-otherwise it falls back to the project provided by [google.FindCredentials].
-
-* Credentials are determined using [Application Default Credentials]. The
-principal must have `roles/monitoring.metricWriter` role granted. If not a
-logged warning will be emitted. Subsequent are silenced to prevent noisy logs.
+The gRPC client is instrumented with Open Telemetry metrics which export to
+Cloud Monitoring by default. More information is available in the
+[gRPC client-side metrics] documentation, including information about
+roles which must be enabled in order to do the export successfully. To
+disable this export, you can use the [WithDisabledClientMetrics] client
+option.
 
 # Storage Control API
 
@@ -394,11 +383,6 @@ and Managed Folder operations) are supported via the autogenerated Storage Contr
 client, which is available as a subpackage in this module. See package docs at
 [cloud.google.com/go/storage/control/apiv2] or reference the [Storage Control API] docs.
 
-[Application Default Credentials]: https://cloud.google.com/docs/authentication/application-default-credentials
-[google.FindCredentials]: https://pkg.go.dev/golang.org/x/oauth2/google#FindDefaultCredentials
-[gRFC/66]: https://github.com/grpc/proposal/blob/master/A66-otel-stats.md
-[gRFC/78]: https://github.com/grpc/proposal/blob/master/A78-grpc-metrics-wrr-pf-xds.md
-[Google Cloud Monitoring]: https://cloud.google.com/monitoring/docs
 [Cloud Storage IAM docs]: https://cloud.google.com/storage/docs/access-control/iam
 [XML POST Object docs]: https://cloud.google.com/storage/docs/xml-api/post-object
 [Cloud Storage retry docs]: https://cloud.google.com/storage/docs/retry-strategy
@@ -408,7 +392,10 @@ client, which is available as a subpackage in this module. See package docs at
 [IAM Service Account Credentials API]: https://console.developers.google.com/apis/api/iamcredentials.googleapis.com/overview
 [custom audit logging]: https://cloud.google.com/storage/docs/audit-logging#add-custom-metadata
 [Storage Control API]: https://cloud.google.com/storage/docs/reference/rpc/google.storage.control.v2
-[metric explorer]: https://console.cloud.google.com/projectselector/monitoring/metrics-explorer
+[Cloud Storage gRPC API]: https://cloud.google.com/storage/docs/enable-grpc-api
 [Direct Connectivity]: https://cloud.google.com/vpc-service-controls/docs/set-up-private-connectivity#direct-connectivity
+[bucket location]: https://cloud.google.com/storage/docs/locations
+[Cloud Storage gRPC docs]: https://cloud.google.com/storage/docs/enable-grpc-api#limitations
+[gRPC client-side metrics]: https://cloud.google.com/storage/docs/client-side-metrics
 */
 package storage // import "cloud.google.com/go/storage"