Fix limit by label key (#3033)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Refactor**
- Updated label key retrieval logic in quota scheduler to enhance
performance and maintain compatibility with new rate limiter interface.
- **Deprecations**
- Added deprecation warnings for fallback usage of the old `GetLabelKey`
method in rate limiting scenarios.
- **Documentation**
- Updated method signatures to reflect changes in label key retrieval
process.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

pkg/policies/flowcontrol/actuators/quota-scheduler/quota-scheduler.go

@@ -345,7 +345,11 @@ func (qs *quotaScheduler) GetSelectors() []*policylangv1.Selector {
 }
 
 func (qs *quotaScheduler) getLabelKey(labels labels.Labels) (string, bool) {
-	labelKey := qs.proto.RateLimiter.GetLabelKey()
+	labelKey := qs.proto.RateLimiter.GetLimitByLabelKey()
+	if labelKey == "" {
+		// Deprecated: Remove in v3.0.0
+		labelKey = qs.proto.RateLimiter.GetLabelKey()
+	}
 	var label string
 	if labelKey == "" {
 		label = "default"

pkg/policies/flowcontrol/actuators/rate-limiter/rate-limiter.go

@@ -393,7 +393,11 @@ func (rl *rateLimiter) TakeIfAvailable(
 		return label, true, 0, 0, 0
 	}
 
-	labelKey := rl.lbProto.Parameters.GetLabelKey()
+	labelKey := rl.lbProto.Parameters.GetLimitByLabelKey()
+	if labelKey == "" {
+		// Deprecated: Remove in v3.0.0
+		labelKey = rl.lbProto.Parameters.GetLabelKey()
+	}
 	if labelKey == "" {
 		label = "default"
 	} else {