ci(tracer): ignore _dd.tracer_kr in snapshot tests (#7644)

The implementation of `_dd.tracer_kr` does not acquire locks to
accurately increment the number of sent spans and kept spans. It it is a
best effort estimation of the number of spans dropped by the tracer. We
should avoid asserting this value in snapshot tests.
 
## Checklist

- [x] Change(s) are motivated and described in the PR description.
- [x] Testing strategy is described if automated tests are not included
in the PR.
- [x] Risk is outlined (performance impact, potential for breakage,
maintainability, etc).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] [Library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
are followed. If no release note is required, add label
`changelog/no-changelog`.
- [x] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/)).
- [x] Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist

- [x] Title is accurate.
- [x] No unnecessary changes are introduced.
- [x] Description motivates each change.
- [x] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes unless absolutely necessary.
- [x] Testing strategy adequately addresses listed risk(s).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] Release note makes sense to a user of the library.
- [x] Reviewer has explicitly acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment.
- [x] 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)
- [x] If this PR touches code that signs or publishes builds or
packages, or handles credentials of any kind, I've requested a review
from `@DataDog/security-design-and-guidance`.
- [x] This PR doesn't touch any of that.

Author: mabdinur

docker-compose.yml

@@ -143,7 +143,7 @@ services:
             - DD_POOL_TRACE_CHECK_FAILURES=true
             - DD_DISABLE_ERROR_RESPONSES=true
             - ENABLED_CHECKS=trace_content_length,trace_stall,meta_tracer_version_header,trace_count_header,trace_peer_service,trace_dd_service
-            - SNAPSHOT_IGNORED_ATTRS=span_id,trace_id,parent_id,duration,start,metrics.system.pid,metrics.system.process_id,metrics.process_id,meta.runtime-id,meta._dd.p.tid,meta.pathway.hash
+            - SNAPSHOT_IGNORED_ATTRS=span_id,trace_id,parent_id,duration,start,metrics.system.pid,metrics.system.process_id,metrics.process_id,meta.runtime-id,meta._dd.p.tid,meta.pathway.hash,metrics._dd.tracer_kr
     vertica:
         image: sumitchawla/vertica
         environment: