[chore][headerssetter] Update docs to clarify how to use "from_context" #31283
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This clarifies potential confusion about how to access metadata using the
from_context
configuration argument.A few months ago I submitted #27465 to remove an obsolete warning, but then a user asked about why their
from_context
isn't working properly with a batch processor. I agree that it's not obvious, so I hope this PR clarifies it enough.I did not try running the update example, so I hope it works :)
cc @jpkrohling