chore(llmobs): add span size telemetry metrics

[Yun-Kim]

What does this PR do?

This PR adds telemetry metrics for span.raw_size and span.size.

Motivation

Plugin Checklist

• Unit tests.
• TypeScript definitions.
• TypeScript tests.
• API documentation.
• CircleCI jobs/workflows.
• Plugin is exported.

Additional Notes

[github-actions]

Overall package size

Self size: 9 MB
Deduped: 101.53 MB
No deduping: 102.05 MB

Dependency sizes

| name | version | self size | total size | |------|---------|-----------|------------| | @datadog/libdatadog | 0.5.0 | 29.83 MB | 29.83 MB | | @datadog/native-appsec | 8.5.1 | 19.26 MB | 19.27 MB | | @datadog/native-iast-taint-tracking | 3.3.0 | 13.77 MB | 13.78 MB | | @datadog/pprof | 5.6.0 | 9.79 MB | 10.16 MB | | @opentelemetry/core | 1.30.1 | 908.66 kB | 7.16 MB | | protobufjs | 7.4.0 | 2.77 MB | 5.42 MB | | @datadog/native-iast-rewriter | 2.8.0 | 2.6 MB | 2.74 MB | | @datadog/native-metrics | 3.1.0 | 1.06 MB | 1.46 MB | | @opentelemetry/api | 1.8.0 | 1.21 MB | 1.21 MB | | import-in-the-middle | 1.13.1 | 117.64 kB | 839.26 kB | | source-map | 0.7.4 | 226 kB | 226 kB | | opentracing | 0.14.7 | 194.81 kB | 194.81 kB | | lru-cache | 7.18.3 | 133.92 kB | 133.92 kB | | pprof-format | 2.1.0 | 111.69 kB | 111.69 kB | | @datadog/sketches-js | 2.1.1 | 109.9 kB | 109.9 kB | | lodash.sortby | 4.7.0 | 75.76 kB | 75.76 kB | | ignore | 5.3.2 | 53.63 kB | 53.63 kB | | istanbul-lib-coverage | 3.2.0 | 29.34 kB | 29.34 kB | | rfdc | 1.4.1 | 27.15 kB | 27.15 kB | | @isaacs/ttlcache | 1.4.1 | 25.2 kB | 25.2 kB | | tlhunter-sorted-set | 0.1.0 | 24.94 kB | 24.94 kB | | dc-polyfill | 0.1.6 | 24.56 kB | 24.56 kB | | shell-quote | 1.8.2 | 23.54 kB | 23.54 kB | | limiter | 1.1.5 | 23.17 kB | 23.17 kB | | retry | 0.13.1 | 18.85 kB | 18.85 kB | | semifies | 1.0.0 | 15.84 kB | 15.84 kB | | jest-docblock | 29.7.0 | 8.99 kB | 12.76 kB | | crypto-randomuuid | 1.0.0 | 11.18 kB | 11.18 kB | | ttl-set | 1.0.0 | 4.61 kB | 9.69 kB | | path-to-regexp | 0.1.12 | 6.6 kB | 6.6 kB | | koalas | 1.0.2 | 6.47 kB | 6.47 kB | | module-details-from-path | 1.0.3 | 4.47 kB | 4.47 kB |

_{🤖 This report was automatically generated by heaviest-objects-in-the-universe}

[codecov]

Codecov Report

Attention: Patch coverage is 4.76190% with 20 lines in your changes missing coverage. Please review.

Project coverage is 79.46%. Comparing base (3bff850) to head (d468db8).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
packages/dd-trace/src/llmobs/telemetry.js	0.00%	14 Missing ⚠️
packages/dd-trace/src/llmobs/writers/spans/base.js	14.28%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5468      +/-   ##
==========================================
+ Coverage   79.39%   79.46%   +0.06%     
==========================================
  Files         500      508       +8     
  Lines       22673    22878     +205     
==========================================
+ Hits        18002    18180     +178     
- Misses       4671     4698      +27     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[datadog-datadog-prod-us1]

Datadog Report

Branch report: yunkim/llmobs-span-size-telemetry
Commit report: 3914b5b
Test service: dd-trace-js-integration-tests

✅ 0 Failed, 802 Passed, 0 Skipped, 16m 9.83s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-03-24 15:08:22

Comparing candidate commit d468db8 in PR branch yunkim/llmobs-span-size-telemetry with baseline commit 3bff850 in branch master.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 915 metrics, 18 unstable metrics.

[sabrenner]

nice job!! just a style thing + small change and then something you added that made me realize we were potentially passing the wrong byte size to the super.append call, but it's something i can fix bc it will have related test changes and is out of scope here i think. otherwise everything lgtm!