chore(telemetry): use enum for telemetry log levels

[romainkomorndatadog]

Makes type-checking more accurate the telemetry writer's add_log() method by specifying an enum.

For example, improperly specifying TELEMETRY_LOG_LEVEL.POTATO instead of .ERROR would result in:

ddtrace/appsec/_iast/_metrics.py:77: error: "Type[TELEMETRY_LOG_LEVEL]" has no attribute "POTATO"  [attr-defined]

This was prompted by #9349 having failures in system-tests due to the use of WARNING instead of WARN.

The telemetry API docs currently specify:

Log level	Remote collection policy
ERROR	Enabled by default, configured by environment variable DD_TELEMETRY_LOG_COLLECTION_ENABLED
WARN	Enabled by default, configured by same environment variable as ERROR
DEBUG	Can be enabled locally by the end user when asked by the support team.

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] Risks are described (performance impact, potential for breakage, maintainability)
• [x] Change is maintainable (easy to change, telemetry, documentation)
• [x] Library release note guidelines are followed or label changelog/no-changelog is set
• [x] Documentation is included (in-code, generated user docs, public corp docs)
• [x] Backport labels are set (if applicable)
• [x] If this PR changes the public interface, I've notified @DataDog/apm-tees.

Reviewer Checklist

• [x] Title is accurate
• [x] All changes are related to the pull request's stated goal
• [x] Description motivates each change
• [x] Avoids breaking API changes
• [x] Testing strategy adequately addresses listed risks
• [x] Change is maintainable (easy to change, telemetry, documentation)
• [x] Release note makes sense to a user of the library
• [x] Author has 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