Ensure user metadata mapped for instance notifications

Fixes bug 1284073

For pollster-originated samples related to instances, we apply a
mapping to the user metadata so as to ensure that there are no
embedded periods in metadata keys and also to exclude any metadata
not matching the configured reserved namespace.

Now this logic is also applied to user metadata for samples derived
from instance-related notifications in order to avoid failures when
persisting these data in mongodb.

(cherry picked from commit ddeb54bb0d2c4203332a5ecc09a4aedfce353a4c)
Change-Id: I77eab59023fa23c5879b05c948bd4cb1db7fa177
Merge "Ensure user metadata mapped for instance notifications" into stable/havana