net: avoid allocation for trace context

[dzbarsky]

Using a string as the context key leads to boxing overhead. See profile: https://www.dropbox.com/s/9u74uvpq8vowwdo/Screenshot%202019-07-30%2012.39.20.png?dl=0

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Patch Set 1:

(3 comments)

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(1 comment)

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Patch Set 3: Run-TryBot+1

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Patch Set 3:

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Patch Set 3: TryBot-Result+1

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Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps:
Within the next week or so, a maintainer will review your change and provide
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Most changes in the Go project go through a few rounds of revision. This can be
surprising to people new to the project. The careful, iterative review process
is our way of helping mentor contributors and ensuring that their contributions
have a lasting impact.

During May-July and Nov-Jan the Go project is in a code freeze, during which
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Patch Set 3:

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Patch Set 3: TryBot-Result+1

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