Support complex attribute values on logs/events in semantic conventions

[lmolkova]

LogRecord attributes are special - they support complex attribute types unlike spans/metrics/resources that support standard attributes only (primitives and arrays of primitives). Note that the latter is the API limitation (not an OTLP one)

In semconv we assume that all attributes are cross-signal and therefore are limited to 'standard' attribute types.

The proposal is to lift this limitation and allow defining complex attributes in semantic conventions with the following limitations:

• they cannot be referenced on spans, metrics, or resources/entities. TBD: check with profiles
  • this can be solved via the semconv policy (given that attribute types are limited on the OTel API surface and not the protocol, we might not want to hardcode it in weaver)
• we should define if/how such attributes would interact with the standard attributes. E.g. is geo.location.lat=x and geo.location.lon=y attributes set on a span are equivalent to geo.location={"lat": x, "lon": y} set on the log record?

This would help with the following problems:

• Logs/Events guidance on what to put to attributes vs body #1651 - we can have one property bag (attributes) to put all the event details into. Standard attributes can be used for correlation across all signals. Complex attributes could be reused across logs preserving semantic meaning (which we don't have today).
• it'd introduce reusable types discussed in Embed functionality v0.2 #1264

Related: open-telemetry/opentelemetry-specification#4334