mod_http_admin_api: Ensure freshness of metrics when in manual mode When in manual collection mode, as recommended for Prometheus, collection needs to be triggered manually, or they would be stale, possibly dating from the start of the server. This might vary per metric depending on how and when the metrics are gathered.

This module logs events where more than a certain amount of CPU time was
spent.

``` lua
log_cpu_threshold = 0.01 -- in seconds, so this is 10 milliseconds
```

Uses the Lua
[`os.clock()`](http://www.lua.org/manual/5.2/manual.html#pdf-os.clock)
function to estimate CPU usage.