Add comment about security guards

Author: NathanielRN

instrumentation/opentelemetry-instrumentation-boto/src/opentelemetry/instrumentation/boto/__init__.py

@@ -235,6 +235,8 @@ def truncate_arg_value(value, max_len=1024):
     if not span.is_recording():
         return
 
+    # Do not trace `Key Management Service` or `Secure Token Service` API calls
+    # over concerns of security leaks.
     if aws_service not in {"kms", "sts"}:
         tags = dict(
             (name, value)