Logs to reuse attribute.[Key]Value and remove log.[Key]Value types

[pellared]

Problem Statement

Right now, the Logs API has similar abstractions to ones in attribute package. For instance, log.KeyValue is almost an equivalent of attribute.KeyValue.

Because of the different types, Logs API provides log.KeyValueFromAttribute which also shows that the intention when the types are used is similar.

What is more with OTEP: Extending attributes to support complex values #4485 we have an agreement that attribute is going to be eventually extended with new types that are now available only in logs signals (ones the addition of new attributes types is stabilized in the specification). Prototype of the new attribute value types: #6809.

Proposed Solution

Remove log.KeyValue (and all related definitions) and use attribute package instead.

The bridges would marshal the values to JSON and use the attribute.String in places where log.MapValue, log.SliceValue and log.BytesValue are currently used: https://github.com/open-telemetry/opentelemetry-go-contrib/blob/main/internal/shared/logutil/convert.go.tmpl.

Alternatives

Keep the log.[Key]Value types to allow usage of the additional value types that are not yet stable for places other than log record attributes and body. However, long-term this would cause unnecessary duplication of types and cause user-confusion why we have different types in the logs API. It would also increase the amount of code that we would have to maintain.

Additional Context

This issue and OTEP: Extending attributes to support complex values #4485 is the main reason why Logs API has not been stabilized yet.

The OTel Go SIG wants very much to simplify the design and prefer to remove the log.[Key]Value types.

We should also update: https://github.com/open-telemetry/opentelemetry-go/blob/main/log/DESIGN.md

[pellared]

Body of LogRecord is optional therefore it is "empty value" by default.

The zero-value of attribute.Value is of INVALID attribute.Type. Existing code (and possibility user-code that we do not own) threat this as an abnormal value

opentelemetry-go/exporters/otlp/otlplog/otlploghttp/internal/transform/log.go

Lines 204 to 207 in 6d9dfea

	default:
	av.Value = &cpb.AnyValue_StringValue{
	StringValue: "INVALID",
	}

as it is explicitly not allowed by the specification:

https://github.com/open-telemetry/opentelemetry-specification/blob/8aa1d27d9cf8c224299dcf0b58bfd5fcaacf88eb/specification/common/README.md?plain=1#L48-L49:

Attribute values of null are not valid and attempting to set a null value is undefined behavior.

In order to go forward, we need an a new EMPTY attribute.Type. So that log.Record.Body() returns it by default. We do not want to have two possibilities of returning an empty value. We want to API to be func (*Record) Body() attribute.Value (and NOT func (*Record) Body() (attribute.Value, bool)).

In order to add the new "empty" attribute type if first needs to be in the stable part of the specification. This is part of open-telemetry/opentelemetry-specification#4485.