rac2: introduce stream token counter provider interface

Introduce the `StreamTokenCounterProvider` interface, which will be used
by replication flow control v2 to access the `TokenCounter` for a given
tenant, store stream.

Note the original interface in the v2 prototype included two additional
methods for metrics gathering which are omitted here.

Resolves: cockroachdb#128010
Release note: None

pkg/kv/kvserver/kvflowcontrol/rac2/BUILD.bazel

@@ -2,7 +2,10 @@ load("@io_bazel_rules_go//go:def.bzl", "go_library")
 
 go_library(
     name = "rac2",
-    srcs = ["token_counter.go"],
+    srcs = [
+        "store_stream.go",
+        "token_counter.go",
+    ],
     importpath = "github.com/cockroachdb/cockroach/pkg/kv/kvserver/kvflowcontrol/rac2",
     visibility = ["//visibility:public"],
     deps = [

pkg/kv/kvserver/kvflowcontrol/rac2/store_stream.go

@@ -0,0 +1,22 @@
+// Copyright 2024 The Cockroach Authors.
+//
+// Use of this software is governed by the Business Source License
+// included in the file licenses/BSL.txt.
+//
+// As of the Change Date specified in that file, in accordance with
+// the Business Source License, use of this software will be governed
+// by the Apache License, Version 2.0, included in the file
+// licenses/APL.txt.
+
+package rac2
+
+import "github.com/cockroachdb/cockroach/pkg/kv/kvserver/kvflowcontrol"
+
+// StreamTokenCounterProvider is the interface for retrieving token counters
+// for a given stream.
+type StreamTokenCounterProvider interface {
+	// Eval returns the evaluation token counter for the given stream.
+	Eval(kvflowcontrol.Stream) TokenCounter
+	// Send returns the send token counter for the given stream.
+	Send(kvflowcontrol.Stream) TokenCounter
+}