Add empty value attribute

[pellared]

Related OTEP: https://github.com/open-telemetry/opentelemetry-specification/blob/main/oteps/4485-extending-attributes-to-support-complex-values.md

To be precise:

opentelemetry-specification/oteps/4485-extending-attributes-to-support-complex-values.md

Lines 40 to 48 in 8aa1d27

	- **Complex attributes** include all other values supported by the `AnyValue` proto,
	such as null (empty) value, maps, heterogeneous arrays, and combinations of those with primitives.
	Byte arrays are also considered complex attributes, as they are excluded from
	the current definition of *standard* attributes.
	- **AnyValue** represents the type of *any* (simple or complex) attribute value on
	the API, SDK, and [proto level](https://github.com/open-telemetry/opentelemetry-proto/blob/42319f8b5bf330f7c3dd4a097384f9f6d5467450/opentelemetry/proto/common/v1/common.proto#L28-L40).
	It's also known as `any` in the [Log data model](https://github.com/open-telemetry/opentelemetry-specification/blob/v1.44.0/specification/logs/data-model.md#type-any)

This is necessary to be able to represent an empty (that is the default) Log Record Body using "Standard Attributes".

This is needed towards stabilization of OTel Go Logs API and SDK (we need it "stable" though; not only "development"); more:

• Logs to reuse attribute.[Key]Value and remove log.[Key]Value types opentelemetry-go#7034

It is also worth mentioning that it is already allowed by OTLP:

message AnyValue {
  // The value is one of the listed fields. It is valid for all values to be unspecified
  // in which case this AnyValue is considered to be "empty".
  oneof value {

Prototype:

• [Prototype] attribute: add complex value types opentelemetry-go#6809

Also fixes #4468

as it removes the portion which said that extending the attribute types is a breaking change as it was already approved via #4485.

[pellared]

CC @open-telemetry/go-maintainers

[lmolkova]

I think we need to start with proto changes and replace comments like https://github.com/open-telemetry/opentelemetry-proto/blob/be5d58470429d0255ffdd49491f0815a3a63d6ef/opentelemetry/proto/trace/v1/trace.proto#L209-L213

// The OpenTelemetry API specification further restricts the allowed value types:
// https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/common/README.md#attribute
// Attribute keys MUST be unique (it is not allowed to have more than one
// attribute with the same key).

(as per https://github.com/open-telemetry/opentelemetry-specification/blob/main/oteps/4485-extending-attributes-to-support-complex-values.md#proto)

we probably need to replace it with comment that they are going to be allowed in 6 months

opentelemetry-specification/oteps/4485-extending-attributes-to-support-complex-values.md

Lines 111 to 112 in 8aa1d27

	- Stable exporters will be prohibited from emitting complex attributes by default on signals
	other than Logs until at least 6 months after this OTEP is merged.

---

Or, at least, leave a note here that complex attributes are coming and their support is in development, nobody should stabilize them in the next 6 month.

[pellared]

we probably need to replace it with comment that they are going to be allowed in 6 months

I feel like this is an orthogonal PR given the changes of https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/common/README.md#attribute are marked with Development status.

Therefore, I think a PR in proto repo can be done in parallel. Are you able to help with it?