Add requirements

Author: carsonip

solutions/observability/apps/transaction-sampling.md

@@ -133,7 +133,7 @@ Tail-based sampling is implemented entirely in APM Server, and will work with tr
 
 Due to [OpenTelemetry tail-based sampling limitations](../../../solutions/observability/apps/limitations.md#apm-open-telemetry-tbs) when using [tailsamplingprocessor](https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/main/processor/tailsamplingprocessor), we recommend using APM Server tail-based sampling instead.
 
-### Tail-based sampling performance [_tail_based_sampling_performance]
+### Tail-based sampling performance and requirements [_tail_based_sampling_performance_and_requirements]
 
 Tail-based sampling, by definition, requires storing events locally temporarily, such that they can be retrieved and forwarded once sampling decision is made.