Cannot discover web.external_url value from API

[bgagnon]

What did you do?
Queried the /api/v1/status endpoint.

What did you expect to see?
The value for the --web.external_url flag.

Alertmanager version
v0.18.0

Note that the flag also missing from the Alertmanager UI. In comparison, the Prometheus status page has a "command line flags" sub-menu that shows all values in a table. I think this UI would be useful for Alertmanager, but that is a separate issue from API discovery.