Fix Timer stopping in http.send

[lukyer]

Fixes incorrect Timer measuring in case of error. Without calling Stop() method when e.g. eval_cancel_error happens the last delta is not added to the accumulated Timer value.

I discovered it by investigating eval_cancel_error when the caller gave up but according to OPA metrics it looked like http.send did not cause it (I would expect near 10s value in timer_rego_builtin_http_send_ns too):

{"counter_rego_builtin_http_send_interquery_cache_hits":1,
"counter_server_query_cache_hit":1,
"timer_rego_builtin_http_send_ns":124580,
"timer_rego_input_parse_ns":4771,
"timer_rego_query_eval_ns":9770617400,
"timer_server_handler_ns":9770659804}