chore(llmobs): [MLOB-2064] add telemetry for custom vs. autoinstrumented span creation

[ncybul]

Adds telemetry to track the number of LLMObs spans created either via auto instrumentation or manual instrumentation.

Checklist

• 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
• The change includes or references documentation updates if necessary
• Backport labels are set (if applicable)

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 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

[github-actions]

CODEOWNERS have been resolved as:

ddtrace/llmobs/_integrations/base.py                                    @DataDog/ml-observability
ddtrace/llmobs/_llmobs.py                                               @DataDog/ml-observability

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-01-24 20:14:02

Comparing candidate commit 9099e10 in PR branch nicole-cybul/add-span-count-telemetry with baseline commit b90fa38 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 394 metrics, 2 unstable metrics.

[Kyle-Verhoog]

nice and simple!