* Tag forwarded spans with custom attributes
Adds a `teleport.forwarded.for` attribute to a resource or
all spans that are forwarded to the auth server. This allows
consumers of the spans to identify where the spans are coming
from and take possible action. In some scenarios it may
be desirable to drop forwarded spans along the collection
process, by tagging them we can provide a way for those
consumers to identify them. It also allows for potentially
identifying a malicious user that may be trying to spam the
telemetry backend with spans.
Part of #12241
rosstimothy
e499d0961e