Update metric attributes for tracing and replays (#15819)

Author: cleptric

develop-docs/sdk/telemetry/metrics.mdx

@@ -460,7 +460,7 @@ We're currently working on a concept for buffering different types of telemetry
 
 #### Tracing
 
-Metrics should be associated with traces. If a metric is recorded during an active span, the SDK should set the `sentry.trace.parent_span_id` attribute to the span id of the span that was active when the metric was collected.
+Metrics should be associated with traces if possible. If a metric is recorded during an active span, the SDK should set the `span_id` property of the metric to the span id of the span that was active when the metric was collected.
 
 #### Replays