Add user_metadata to network samples

Previously 'user_metadata' is not included in ceilometer network samples,
while it is in other samples. This causes network samples not triggering
alarms heat created.

Change-Id: Iafb4c42955fceaaf20fef1f9580ddb1331a0c724
Closes-Bug: 1369500
(cherry picked from commit cca6fcd89e0044f310f6b05c62665302582e14e1)
Merge "Add user_metadata to network samples" into stable/juno