mgr/prometheus: Cast collect_timeout (scrape_interval) to float

mgr/prometheus: Cast collect_timeout (scrape_interval) to float

If set by user scrape_interval option is returned as non-float by get_localized_module_option.
Metric cache timeout comparison always returns true and data is never refreshed.

Fixes:  https://tracker.ceph.com/issues/40997
Signed-off-by: Ben Meekhof <bmeekhof@umich.edu>
(cherry picked from commit 26a74a0d83e068b0bb762c4c7066b4b195187e94)

Conflicts:
- path: src/pybind/mgr/prometheus/module.py
  comment: get_localized_module_option() in master, get_localized_config() in luminous