Support the OTEP-4947 trace-flags byte

[szegedi]

When we started working on the Otel Thread Context support, the record had a reserved field, but when OTEP-4797 was finalized, it became a trace-flags byte (offset 25), which OTEP-4947 defines as the
W3C trace-flags. We need to catch up with this change.

API change

new ThreadContext(traceId, spanId, attributes?)
new ThreadContext(traceId, spanId, traceFlags?, attributes?)

traceFlags goes third, next to the ids it describes, rather than being appended after attributes. This will break callers passing attributes positionally; TypeScript rejects it at compile time, and at runtime an array in slot 3 throws
traceFlags must be an integer in 0..255. It's okay to break callers, as dd-trace-js is the only known consumer at the moment and we can make the matching change when we release pprof-nodejs.

Any integer in 0..255 is accepted rather than masked to the two bits W3C currently defines, because W3C requires unknown flag bits to be propagated. Absent / undefined / null means 0, which is what OTEP-4947 prescribes when no flags are known.

setTraceFlags()

A constructor argument alone would not be enough. dd-trace's sampling decision is deferred, so it should be possible to set the value later.

[github-actions]

Overall package size

Self size: 2.58 MB
Deduped: 3.28 MB
No deduping: 3.28 MB

Dependency sizes

| name | version | self size | total size | |------|---------|-----------|------------| | pprof-format | 2.3.1 | 504.33 kB | 504.33 kB | | source-map | 0.8.0 | 185.66 kB | 185.66 kB | | node-gyp-build | 4.8.4 | 13.86 kB | 13.86 kB |

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