Tolerate absent recorded_at on older mongo/db2 samples

Fixes bug 1284608

Prevents failure of a samples query against a mongo-like instance
(i.e. either mongodb or db2) selecting metering data that predates
the advent of the recorded_at attribute for samples:

  Ia0ff8bd07fd811fe8d3050d30971a05a277798d0

Change-Id: If480321923e9166583bbea91c3878950bafbedf1
Merge "Tolerate absent recorded_at on older mongo/db2 samples"