fix: Check for no microseconds in timestamp for pubsub message handler

[koenmtb1]

Fixes #160

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[koenmtb1]

I've verified this to work locally by overwriting the _message_handler with this version. It no longer throws any errors and because I had retries turned on it properly started processing any event that had this issue.

[exaby73]

@koenmtb1 could you add tests for this?

[inlined]

Additionally, please fix the formatting errors

[inlined]

Also, Google servers don't typically send microseconds that I'm aware of. Are you seeing problems in production or in the emulator?

[dblake10]

@inlined - Having encountered this error today, I can confirm that I'm seeing problems with this in production. @koenmtb1 - Following up on the testing and formatting requests above (from 9 to 10 weeks ago). It would be nice to get this merged so that others don't encounter this same issue.

[exaby73]

Closing in favor of https://github.com/firebase/firebase-functions-python/pull/204