daemon: instrument the gateway and api HTTP handlers

[ghost]

So that we can graph them using Prometheus. See #1763 and #1422.

This gets us the following metrics:

# HELP http_requests_total Total number of HTTP requests made.
# TYPE http_requests_total counter
http_requests_total{code="200",handler="api",method="get"} 54
http_requests_total{code="200",handler="gateway",method="get"} 60
http_requests_total{code="304",handler="gateway",method="get"} 30
# HELP http_request_duration_microseconds The HTTP request latencies in microseconds.
# TYPE http_request_duration_microseconds summary
http_request_duration_microseconds{handler="api",quantile="0.5"} 593.504
http_request_duration_microseconds{handler="api",quantile="0.9"} 852.337
http_request_duration_microseconds{handler="api",quantile="0.99"} 1054.733
http_request_duration_microseconds_sum{handler="api"} 35359.53
http_request_duration_microseconds_count{handler="api"} 54
http_request_duration_microseconds{handler="gateway",quantile="0.5"} 21147.23
http_request_duration_microseconds{handler="gateway",quantile="0.9"} 337680.504
http_request_duration_microseconds{handler="gateway",quantile="0.99"} 2.236680159e+06
http_request_duration_microseconds_sum{handler="gateway"} 2.1692309521000005e+07
http_request_duration_microseconds_count{handler="gateway"} 90
# HELP http_request_size_bytes The HTTP request sizes in bytes.
# TYPE http_request_size_bytes summary
http_request_size_bytes{handler="api",quantile="0.5"} 80
http_request_size_bytes{handler="api",quantile="0.9"} 80
http_request_size_bytes{handler="api",quantile="0.99"} 80
http_request_size_bytes_sum{handler="api"} 4320
http_request_size_bytes_count{handler="api"} 54
http_request_size_bytes{handler="gateway",quantile="0.5"} 587
http_request_size_bytes{handler="gateway",quantile="0.9"} 617
http_request_size_bytes{handler="gateway",quantile="0.99"} 645
http_request_size_bytes_sum{handler="gateway"} 52789
http_request_size_bytes_count{handler="gateway"} 90

[whyrusleeping]

woo!