Pass a custom registry to MetricsHandler

[ioggstream]

I expect

to be able to pass a custom registry to MetricsHandler

Instead

I can't

I'd do it

passing the registry to start_http_server

[brian-brazil]

Dupe of #214

[ioggstream]

@brian-brazil as there's probably something I'm missing, it's better to split the patch in two:

• this one for passing the custom registry
• the other one for the multiprocess stuff.