`metrics_path` is silently ignored when used together with *-notation

Bug Description

When a scrape job has a custom metrics_path AND uses star-notation, then the other side of the relation (consumer, i.e. the prom that scrapes) has the custom metrics_path ignored, and uses the default one.

On one hand, multiple units may have different hostnames but the exact same path prefix. In those cases the star-notation should still work.
On the other hand, with ingress-per-unit, each unit will have a different path so it should be possible to combine it with star-notation.

EDIT

Put differently, a job's target hostname has no effect, if the external_hostname= kwarg is not passed to MetricsEndpointProvider.

Still, after passing external_hostname, there is some duplication (apart from ingress nor path not being picked up):

> $ curl 10.1.13.180:9090/api/v1/targets| jq | grep "scrapeUrl"
        "scrapeUrl": "http://prometheus-0.prometheus-endpoints.test-external-url-f4ew.svc.cluster.local:9090/metrics",
        "scrapeUrl": "http://prometheus-0.prometheus-endpoints.test-external-url-f4ew.svc.cluster.local:80/metrics",
        "scrapeUrl": "http://prometheus-1.prometheus-endpoints.test-external-url-f4ew.svc.cluster.local:80/metrics",
        "scrapeUrl": "http://localhost:9090/metrics",

To Reproduce

Pass the following job to PromScrapeProvider:

{"metrics_path": "/foo/metrics", "static_configs": [{"targets": [f"*:{self._port}"]}]}

Environment

N/A

Relevant log output

scrape_jobs: '[{"metrics_path": "/metrics", "static_configs": [{"targets": ["*:9090"]}]}]'

Additional context

Noticed this while working on ingress (#349).

canonical / prometheus-k8s-operator