What does this PR do?

Add registry parameter to start_http_server, so you can serve data
from a custom registry.

How it's implemented

Added:

• the optional named parameter registry to start_http_server
• create a MetricHandler tied to the passed registry via MetricHandler.factory

How do you use it

Create a new or a multiprocess registry
Pass registry=myregistry to start_http_server.

from prometheus_client import multiprocess, start_http_server
from prometheus_client import Counter
...
# Serve myregistry metrics.
myregistry = CollectorRegistry()
start_http_server(8000, registry=myregistry)

# Server core.REGISTRY metrics
start_http_server(8001) 
...