```
> require('http').globalAgent.sockets
{ 'HOST:PORT:': [{ ... }, { ... }] }
```
- `new URL('http://HOST:PORT:')` throws
-> test suite `open_sockets`
- gauges were not cleaned up after no agents were present for a given
key. This lead to continuous emission of the previous connection
count for a given hostname, following the completion of all requests.
-> test suite `when all requests complete`
Metrics.initialize() and Metrics.configure() will configure the default
Prometheus registry with default "app" and "host" labels. The configured
prom-client module is available as Metrics.prom.
Version 3.2.0 crashes on startup because it calls buildPromKey() without
arguments. To avoid this kind of obvious bug to happen again, I added
some basic acceptance tests.
