`pathway web-dashboard` should support waiting until metrics database appears

[lpsinger]

Is your feature request related to a problem? Please describe.
If there is not yet a metrics database present, then pathway web-dashboard halts immediately with the error message FileNotFoundError: No metrics database found. This may be useful default behavior, but for deployment purposes it is easier if you can start both pathway and its web dashboard at the same time.

Describe the solution you'd like
A command-line option for pathway web-dashboard that causes it to wait until a metrics database file appears.

Describe alternatives you've considered
This workaround: https://git.ligo.org/ultra-swift/shigawire/-/blob/ed07dbb91e4e7de244be53d88b8b131643d25eaf/shigawire/dashboard.py#L11-31

Additional context
Add any other context or screenshots about the feature request here.

[zxqfd555]

Thank you for the report.

This case definitely makes sense, we will support it in the next release.