TODOs for OM compatibility in Prom python client

[Lee Calcote]

Brian,

Thanks for your help on comprehending python 2 vs 3 encode/decode on UTF-8 validation. PR for the first of your TODOs is up for your review - https://github.com/prometheus/client_python/pull/312.

We used \U00010000 as an invalid UTF-8 test case and updated the label parser to flag on characters with ordinal’s greater than 65535.

Cheers,

Lee