fix(logging): allow X-Cloud-Trace-Context fields to be optional

[freelerobot]

Fixes: #1590

Changes according to spec:

1. trace, spanID and traceSampled fields in X-Cloud-Trace-Context can be optional (see context/trace_context.proto)
2. TraceSample should default to false (verified with nwang@)
3. Trace/span IDs should still be included in log contexts even when traceSampled is false, because "A non-sampled trace value is still useful as a request correlation identifier." (LogEntry spec)

Details:
Intended Use case

The following header specifications by users are now recognized:

[odeke-em]

Thank you for this change @nicoleczhu! I've added some suggestions, please take a look.

[odeke-em]

Please restore (and adjust) the length checks to ensure that we have at least 5 components otherwise we'll encounter crashes later on.

[freelerobot]

I double checked this one, given the new regex pattern/FindStringSubmatch, it will always return 6 components even if string s is "" or very wrongly formatted. See sample code here: https://play.golang.org/p/-adjLHwv1VD

[odeke-em]

That regex isn't very legible, I'd say just for the sake of maintaining invariants and easily inspectable behavior, let's enforce those length counts. A single change to that regex without the appropriate checks can cause later surprises.

[freelerobot]

That's a good point. How about I add descriptive comments to the regex itself, so folks changing the regex don't encounter surprises.

I'd really prefer disclosing in comments, rather than enforcing a if(true) type of check in code. I think checking length might also confuse folks looking at the regex in the future, in that it suggests the regex pattern could return various lengths (when it never will).

[odeke-em]

Judgement call/preference :) but personally I'd ensure that count and even panic if not

[freelerobot]

Update:

1. I broke apart the regex, adding comments for readability.
2. I used non-capturing groups this time, so the matched indeces are more intuitive (1,2,3) rather than (1,3,5).

Thanks for the style feedback on this!

[codyoss]

LGTM