instance_ip_grouping_key does not work on macOS

[darkclouder]

macOS 11.1
Python 3.8.5
prometheus-client 0.9.0

To reproduce:

from prometheus_client import instance_ip_grouping_key

instance_ip_grouping_key()

What happens?

metrics.py:10: in <module>
    INSTANCE = instance_ip_grouping_key()
lib/python3.8/site-packages/prometheus_client/exposition.py:379: in instance_ip_grouping_key
    s.connect(('localhost', 0))
E   OSError: [Errno 49] Can't assign requested address

[csmarchbanks]

Thank you for your report, it is certainly an issue, and there is even a test that is ignored on macOS: https://github.com/prometheus/client_python/blob/master/tests/test_exposition.py#L379

I don't have a macOS environment, but would be happy to review a PR to enable macOS.

[AceFire6]

@csmarchbanks Does this look like an acceptable solution?

Edit: I've submitted a PR #687

It's adapted from this StackOverflow answer: https://stackoverflow.com/a/28950776

def instance_ip_grouping_key():
    """Grouping key with instance set to the IP Address of this host."""
    with closing(socket.socket(socket.AF_INET, socket.SOCK_DGRAM)) as s:
        s.connect(('10.255.255.255', 1))
        return {'instance': s.getsockname()[0]}

It works on my M1 MacbookPro:

>>> from contextlib import closing
>>> 
>>> def instance_ip_grouping_key():
...     """Grouping key with instance set to the IP Address of this host."""
...     with closing(socket.socket(socket.AF_INET, socket.SOCK_DGRAM)) as s:
...         s.connect(('10.255.255.255', 1))
...         return {'instance': s.getsockname()[0]}
... 
>>> instance_ip_grouping_key()
{'instance': '192.168.0.189'}