Metrics API endpoint: include UNKNOWN sources

[rdimaio]

Description

the new API does not include the recently discovered UNKNOWN sources, which includes any subscriptions where the source was not resolved yet.

Reference: https://gitlab.cern.ch/atlas-adc-ddm/flux-rucio/-/issues/10#note_7726087

       "UNKNOWN:NET2": {
                "dst": "NET2", 
                "files": {
                        "queued": {
                                ...
                                "Production Output": {
                                        "latest": 119499, 
                                        "timestamp": "2024-03-13T14:55:03"
                                }
                        }, 
                        "queued-total": 119559
                }, 
                "src": "UNKNOWN"
        },

Motivation

No response

Change

No response