kv: extract TenantSpanConfig 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 `TenantSpanConfig`. 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

@@ -3692,6 +3692,26 @@ service TenantUsage {
   rpc TokenBucket (TokenBucketRequest) returns (TokenBucketResponse) {}
 }
 
+// TenantSpanConfig provides RPCs for secondary tenants to operate on span
+// configs, such as reading and updating them.
+service TenantSpanConfig {
+  // GetSpanConfigs is used to fetch the span configurations over a given
+  // keyspan.
+  rpc GetSpanConfigs(GetSpanConfigsRequest) returns (GetSpanConfigsResponse) {}
+
+  // GetAllSystemSpanConfigsThatApply is used to fetch all system span
+  // configurations that apply over a tenant's ranges.
+  rpc GetAllSystemSpanConfigsThatApply(GetAllSystemSpanConfigsThatApplyRequest) returns (GetAllSystemSpanConfigsThatApplyResponse) {}
+
+  // UpdateSpanConfigs is used to update the span configurations over given
+  // keyspans.
+  rpc UpdateSpanConfigs(UpdateSpanConfigsRequest) returns (UpdateSpanConfigsResponse) {}
+
+  // SpanConfigConformance is used to determine whether ranges backing the given
+  // keyspans conform to span configs that apply over them.
+  rpc SpanConfigConformance(SpanConfigConformanceRequest) returns (SpanConfigConformanceResponse) {}
+}
+
 // Batch and RangeFeed service implemented by nodes for KV API requests.
 service Internal {
   rpc Batch (BatchRequest) returns (BatchResponse) {}

pkg/server/server.go

@@ -992,6 +992,9 @@ func NewServer(cfg Config, stopper *stop.Stopper) (serverctl.ServerStartupInterf
 	if err := kvpb.DRPCRegisterTenantUsage(drpcServer, node); err != nil {
 		return nil, err
 	}
+	if err := kvpb.DRPCRegisterTenantSpanConfig(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