kv: extract TenantUsage and register with DRPC server

This is a follow-up to #145195, where we extracted `KVBatch` from the
`Internal` service. Here, we do the same with `TenantUsage`. The TL;DR is
that smaller services are easier to maintain and can be more smoothly
migrated to DRPC.

We also enable this service on the DRPC server. This is controlled by
`rpc.experimental_drpc.enabled` (off by default).

This change is part of a series and is similar to: #146926

Note: This only registers the service; the client is not updated to use the
DRPC client, so this service will not have any functional effect.

Epic: CRDB-48925
Release note: None

Author: shubhamdhama

pkg/kv/kvpb/api.proto

@@ -3684,6 +3684,14 @@ service TenantService {
   rpc GetRangeDescriptors(GetRangeDescriptorsRequest) returns (stream GetRangeDescriptorsResponse) {}
 }
 
+// TenantUsage provides RPCs for secondary tenants to report resource
+// consumption and get Request Units available.
+service TenantUsage {
+  // TokenBucket is used by tenants to obtain Request Units and report
+  // consumption.
+  rpc TokenBucket (TokenBucketRequest) returns (TokenBucketResponse) {}
+}
+
 // Batch and RangeFeed service implemented by nodes for KV API requests.
 service Internal {
   rpc Batch (BatchRequest) returns (BatchResponse) {}

pkg/server/server.go

@@ -989,6 +989,9 @@ func NewServer(cfg Config, stopper *stop.Stopper) (serverctl.ServerStartupInterf
 	if err := kvpb.DRPCRegisterTenantService(drpcServer, node.AsDRPCTenantServiceServer()); err != nil {
 		return nil, err
 	}
+	if err := kvpb.DRPCRegisterTenantUsage(drpcServer, node); err != nil {
+		return nil, err
+	}
 	kvserver.RegisterPerReplicaServer(grpcServer.Server, node.perReplicaServer)
 	if err := kvserver.DRPCRegisterPerReplica(drpcServer, node.perReplicaServer); err != nil {
 		return nil, err