namespace/subsystem are not taken into consideration in multiproc mode

[atollena]

The namespace and subsystem kwargs add prefixes to metric name when running normally. However, when using the multiprocess mode of the library, those prefixes are ignored.

Here is a small program to reproduce this problem:

from prometheus_client import Counter, CollectorRegistry, start_http_server, generate_latest, multiprocess
import time

registry = CollectorRegistry()
c = Counter('test', 'Test', namespace='foo', subsystem='bar', registry=registry)
c.inc()

multiprocess.MultiProcessCollector(registry)
print(generate_latest(registry).decode())

Running this program prints:

# HELP foo_bar_test Test
# TYPE foo_bar_test counter
foo_bar_test 1.0
# HELP test Multiprocess metric
# TYPE test counter
test 1.0

[atollena]

@brian-brazil full_name could simply be passed instead of just name at https://github.com/prometheus/client_python/blob/master/prometheus_client/core.py#L537 and https://github.com/prometheus/client_python/blob/master/prometheus_client/core.py#L539, but it's not obvious to me that it is the right fix.

I'll be happy to make this change or another if you think that it is reasonable.

[brian-brazil]

That sounds like the right fix, and a PR would be great.