Unnecessary conversion of trace context to base64 in OTLP exporters

[seemk]

OTLP pipeline converts trace and span IDs from hex to base64 only for these to be decoded back from base64 in protobuf.js when serializing the payload.

hexToBase64 should be replaced with hexToBinary which converts the hex strings to Uint8Array.

[dyladan]

When this was written our protobuf serialization library took base64 input in order to produce binary output, not Uint8Array. We've since changed a lot about the way proto is handled so I wouldn't be surprised if both work now.

[seemk]

When this was written our protobuf serialization library took base64 input in order to produce binary output, not Uint8Array. We've since changed a lot about the way proto is handled so I wouldn't be surprised if both work now.

Yup, Uint8Array can be passed to protobufjs and it's quite efficient.

Fix at #4343