mgr/ActivePyModules: fix dangling string_view in get_perf_schema_python

mgr/ActivePyModules: fix dangling string_view in get_perf_schema_python

prev_key_name and key_name variables were declared as std::string_view but
prev_key_name was assigned from key_name, which points to
key_name_without_counter's buffer. This buffer is overwritten each iteration,
causing prev_key_name and key_name to become a dangling pointers.

This results in corrupted comparisons and can cause assertion failures like
("cursor != root" in PyFormatter::close_section) or segmentation faults
when the corrupted string is passed back to telemetry.

Fixes: https://tracker.ceph.com/issues/74286
Signed-off-by: Nitzan Mordechai <nmordech@ibm.com>