topdown: Pass metrics to built-ins and record http.send latency

Previously the built-in functions had no way to record metrics for
performance monitoring or other purposes. With this change, built-in
functions can manipulate the evaluation metrics. Initially, only the
http.send function has been updated to report latency.

Fixes #2034

Signed-off-by: Torin Sandall <torinsandall@gmail.com>