common: fix md_config_cacher_t

common: fix md_config_cacher_t

In its get_tracked_conf_keys() member function, the
cacher (in the existing code) initializes a static
function-block variable ('keys'), and uses it for
registering the observer.

But the cacher is instantiated on the type of
the configuration value. Thus, multiple cacher
objects for which the configuration values are
of the same type - share the static 'keys'. Only
one of the observers is registered.

Note that the code could have been simplified
somewhat, if the signature of the
get_tracked_conf_keys() function
was changed to return 'const char* const *'.

Fixes: https://tracker.ceph.com/issues/69236
Signed-off-by: Ronen Friedman <rfriedma@redhat.com>