fix(tvix/tracing): make cargo check and clippy happy r/8310

In case the otlp feature is not enabled, these generate warnings during
`cargo check`.
Fix by moving some imports into their functions, or using the
fully-qualified name (and one #[allow(unused_mut)])

Change-Id: I5afd89dcd4c772b6002cebdd5d0469932eacfdac
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11873
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: Connor Brewster <cbrewster@hey.com>
Reviewed-by: Simon Hauser <simon.hauser@helsinki-systems.de>

0 files changed, 0 insertions, 0 deletions