Fix the following CI failures:
https://gitlab.ddbuild.io/DataDog/apm-reliability/dd-trace-py/-/jobs/774636553

Reproduction:
```
from ddtrace.appsec._metrics import _set_waf_init_metric
```

- `ddtrace/appsec/_metrics` defines
[_set_waf_init_metric](https://github.com/DataDog/dd-trace-py/blob/130b69b367e22311fa5fea7e3cc0910396e968c4/ddtrace/appsec/_metrics.py#L48-L49)
- `ddtrace/appsec/_metrics` imports
[ddtrace.appsec._proccessors](https://github.com/DataDog/dd-trace-py/blob/130b69b367e22311fa5fea7e3cc0910396e968c4/ddtrace/appsec/_metrics.py#L4-L5)
- `ddtrace.appsec._proccessors` imports
[_set_waf_init_metric](https://github.com/DataDog/dd-trace-py/blob/130b69b367e22311fa5fea7e3cc0910396e968c4/ddtrace/appsec/_processor.py#L443-L444)
- Boom: Circular import

The fix here is to avoid importing `ddwaf` from
`ddtrace.appsec._processor` in `ddtrace.appsec._metrics`. Instead we can
import ddwaf directly from `ddtrace.appsec._ddwaf`.

This issue does not occur if the `ddtrace.appsec._processor` module is
imported before `ddtrace.appsec._metrics`. This is why we do not see
this error in most of our appsec tests. However the order of imports is
not guaranteed by the slots check. This explains the flakiness in this
job.

## 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
- [x] 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)

(cherry picked from commit 67e1c46)