common/LogEntry: Add log level to str helper for fmt::formatter<LogEntry>

common/LogEntry: Add log level to str helper for fmt::formatter<LogEntry>

The Ceph cluster logs were missing the string equivalent [INF|WRN|ERR|DBG]
representation of the 'prio' field. This was broken since the introduction
of commit 2901943618f071060f72d69cf632a4097e439df4 of
PR: https://github.com/ceph/ceph/pull/47830. This probably caused false
positives in teuthology testing and particularly for those tests that check
for cluster badness by parsing the cluster logs.

The fix involves adding a static helper function to the LogEntry struct.
This function returns the string appropriate representation of the log
level similar to the operator<<() for LogEntry.

Fixes: https://tracker.ceph.com/issues/64314
Signed-off-by: Sridhar Seshasayee <sseshasa@redhat.com>