[1.7] Tracing fields should be at the root

[webmat]

Backports the following commits to 1.7:

• Add notice to the tracing field set, about not nesting field names. (Add notice to the tracing field set, about not nesting field names. #1162)
• Tracing fields should be at top level in Beats artifact (Tracing fields should be at top level in Beats artifact #1164)