Document redpanda.otel_traces topic billing

   - Clarify that Serverless clusters are not billed for traces topic usage
   - Note that Dedicated/BYOC clusters follow standard billing for traces

Author: JakeSCahill

modules/billing/pages/billing.adoc

@@ -140,6 +140,8 @@ Pricing per MCP server depends on the compute units you allocate. The cost of a
 
 NOTE: Compute units for Remote MCP use the same definition and rates as those for Redpanda Connect.
 
+Remote MCP servers automatically emit OpenTelemetry traces to the xref:ai-agents:mcp/remote/monitor-activity.adoc#opentelemetry-traces-topic[`redpanda.otel_traces` topic]. For Serverless clusters, usage of this system-managed traces topic is not billed. You will not incur ingress, egress, storage, or partition charges for trace data. For Dedicated and BYOC clusters, standard billing metrics apply to the traces topic.
+
 == Support plans
 
 All organizations in Redpanda require one of the following support plans: