b3170af changed the DstLabels api, but the bench test was not updated
accordingly.
Furthermore, since bench tests require a nightly rust version, we've
avoided running them in CI. This makes it easy for these tests to break, however.
This updates the benches/record.rs. Additionally, in CI, we pin the rust nightly'
version to a known-good version so that we can reliably run these bench test
without the fear of external changes breaking our build.
db2478f5a2