specify span id

Author: chargome

develop-docs/sdk/telemetry/metrics.mdx

@@ -90,7 +90,7 @@ Integers should be a 64-bit signed integer, while doubles should be a 64-bit flo
 
 `span_id`
 
-: **String, optional**. The span id for the metric.
+: **String, optional**. The span id of the span that was active when the metric was emitted. The value should be 8 random bytes encoded as a hex string (16 characters long). The span id should be grabbed from the current active span in the SDK.
 
 `unit`