perf(tracing): use get_local_item instead of get_item (#14404)

`core.get_item`/`ExecutionContext.get_item` will search through the
hierarchy of parents until it finds the key (or cache miss on all
parents and returns the default).

Most usage of context set_item/get_item are intended to be for the
currently actively context only, and don't need to search for an item
potentially on the parent.

When profiling `get_item` can be up to 5-6x slower than `get_local_item`
especially for cache misses.

This change updates the `ddtrace/_trace/trace_handlers.py` usage to only
use `get_local_item` instead.


## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [ ] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
- Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

Author: brettlangdon