fix: encode OTLP trace identifiers as hex

[incidere]

Fixes #50.

OTLP's JSON mapping represents trace and span identifiers as hexadecimal strings instead of protobuf's default base64.

This change:

• converts traceId, spanId, and parentSpanId after protobuf JSON encoding;
• preserves standard base64 encoding for unrelated bytes fields;
• adds a regression test covering Span, Link, a parent span, and an unrelated bytes attribute.

Tests:

• zig build sdk-test -Dtarget=x86_64-linux-musl — 290 tests passed
• zig fmt --check opentelemetry-sdk/src/otlp.zig

[linux-foundation-easycla]

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[agagniere]

I would prefer to wait for Arwalk/zig-protobuf#176 to do it cleanly

[inge4pres]

Thanks for sending this PR @incidere 🙏🏼

I agree with @agagniere that the proposed approach, while functionally working, can be better addressed at a lower level in the protobuf JSON decoder (real fix is in Jacob's PR Arwalk/zig-protobuf#176).

I liked that you added tests, would love to see them added also for LogRecord (and also Exemplars once implemented), but only after we have bumped our zig-protobuf depedency.

[inge4pres]

Zig 0.16.0 has a std.Io.Writer.printHex function that converts to Hex digits for us
https://ziglang.org/documentation/0.16.0/std/#std.Io.Writer.printHex