(PDB-653) metrics update on PDB start

Previously the DLO metrics only updated on a failed command submission, which
meant restarting PDB would blank the metrics until the next failure. This patch
initializes DLO metrics on PDB startup if the DLO directory, which is created
on first failure, already exists.
Merge pull request #1079 from wkalt/ticket/master/pdb-653-metrics-not-updating

(PDB-653) metrics update on PDB start