Cherry-pick #12743 to 7.3: Fix race condition on error returned after context cancellation #12849
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Cherry-pick of PR #12743 to 7.3 branch. Original message:
After #11981 docker client watching event uses the context provided by
the metricset wrapper. This produces a race condition on error
reporting: the errors channel will receive a context cancelled error when
the context is done, so both paths can be chosen by the select. If the
errors one is chosen, an error will be reported and a reconnect will be
attempted, that will fail.
Alternativelly we could have created another context and cancel it after
the reporter is done, in a similar fashion to what was done before #11981,
but then we would be breaking the chain of derived contexts.
Fixing it this way at least by now to avoid the flaking errors that this
is causing.