In T340552, the official PHP OpenTelemetry client was effectively
rejected for inclusion in MediaWiki due to its size. Implement a minimal
tracing library instead that eschews conformance with the OTEL client
specification in favor of simplicity, while remaining capable of
emitting trace data in OTLP format and thus retaining compatibility with
any ingestion endpoint capable of handling OTLP.
In its current state, the library supports a basic feature set that
should be sufficient for basic tracing integration:
* Span creation, inclusive span activation and automatic parent span
assignment,
* Span attributes and span kinds,
* Basic resource (process/request)-level metadata generation,
* Data export over OTLP.
Additional functionality, such as trace propagation, can then be
incrementally added to the library.
Bug: T340552
Change-Id: Ibc3910058cd7ed064cad293a3cdc091344e66b86
16ec1a3703