Create attribute.Value type for []byte values

[jmacd]

Problem Statement

When a user has non-utf8 data that they wish to include in their telemetry, they face limited options. It is not valid to use a string representation for all []byte values, because OTLP has a protobuf representation which specifies valid utf8 for strings.

Therefore, when a user has []byte values, they cannot use a string w/o calling e.g., strings.ToValidUTF8() and modify the original data. At first I thought there might be a specification document that forbids OTel APIs from supporting byte slices, but I looked and didn't find anything, so it looks like no spec changes are needed.

Proposed Solution

The OTLP protocol has an AnyValue member for binary data. We have no API-level way to enter this data. Proposing to create Bytes(string, []byte) KeyValue and BytesValue([]byte) Value members in the attribute package.

Alternatives

As discussed in open-telemetry/opentelemetry-specification#3421, if invalid utf8 is allowed into a gRPC-based export pipeline, the protobuf library obligates considering invalid utf8 as an error, which causes whole batches of telemetry to be rejected. So, the alternative is to consider invalid utf8 an error, and require users to validate their inputs.

Prior Art

The Zap logging library has a constructor named zap.Binary(string, []byte) which is exactly what I'd like, except I consider it more Go-idiomatic to name this Bytes().

[MrAlias]

Please see #5089. This was already proposed and, as pointed out here, the specification SIG has decided this is not something we want to do.

[jmacd]

I followed the linked issue to open-telemetry/opentelemetry-specification#3950, and it appears unresolved. If you do not object, I think we should leave this open or close this and re-open #5089. I believe users have a legitimate request, with the zap.Binary method and the presence of an OTLP bytes_value.

[pellared]

I believe users have a legitimate request, with the zap.Binary method and the presence of an OTLP bytes_value.

Just for clarification, log attributes do support []byte: https://pkg.go.dev/go.opentelemetry.io/otel/log#Bytes

Bytes is not supported for other signals (tracing, metrics).

[jmacd]

@pellared Surprising. I guess users can't use the same attributes for logs and spans. I still think the specification should move to allow the byte-valued slices, but am not going to champion this.