Merge pull request #16860 from joeljames/patch-3

Add head sampling trace session details

src/content/docs/distributed-tracing/concepts/how-new-relic-distributed-tracing-works.mdx

@@ -49,7 +49,9 @@ We have a few sampling strategies available:
 
 ### Head-based sampling (standard distributed tracing) [#head-based]
 
-With the exception of our [Infinite Tracing](#tail-based) feature, most of our tracing tools use a head-based sampling approach. This applies filters to individual spans before all spans in a trace arrive, which means decisions about whether to accept spans are made at the beginning (the "head") of the filtering process. We use this sampling strategy to capture a representative sample of activity while avoiding storage and performance issues.
+With the exception of our [Infinite Tracing](#tail-based) feature, most of our tracing tools use a head-based sampling approach. This applies filters to individual spans before all spans in a trace arrive, which means decisions about whether to accept spans are made at the beginning (the "head") of the filtering process. We use this sampling strategy to capture a representative sample of activity while avoiding storage and performance issues. 
+
+Once the first span in a trace arrives, a session is opened and maintained for 90 seconds. With each subsequent arrival of a new span for that trace, the expiration time is reset to 90 seconds. Traces that have not received a span within the last 90 seconds will automatically close. The trace summary and span data are only written when a trace is closed.
 
 Here are some details about how head-based sampling is implemented in our standard distributed tracing tools: