Add OTEL Semantic Conventions support

[Mpdreamz]

Summary

• Add OTel semantic convention mappings with bidirectional ECS↔OTel support. Parse otel mappings from the ECS 9.x YAML spec, generate bidirectional mapping tables, and expose AssignOTelField, SemConv constants, and OTelMappings dictionaries. OTel attributes are stored in an Attributes passthrough dictionary and serialized under an attributes object alongside standard ECS fields.
• Consolidate Labels and Metadata into the single Attributes passthrough. Both properties are now [Obsolete]; serialization merges all three into a single "attributes" JSON key (Attributes wins over Metadata wins over Labels). Deserialization reads "labels", "metadata", and "attributes" JSON keys all into Attributes. The ES base component gains an attributes: passthrough mapping.

OTEL support

Parse otel mappings from the ECS 9.x YAML spec, generate bidirectional
mapping tables (ECS <-> OTel), and support reading/writing OTel attributes
alongside standard ECS fields.

ECS fields are always serialized as ECS fields. OTel semantic convention
attributes are stored in the Attributes passthrough dictionary and
serialized under an attributes object.

Setting OTel attributes

Use SemConv constants with AssignOTelField to set an OTel attribute
that is stored in Attributes and simultaneously mapped to its ECS
property:

var doc = new EcsDocument();
doc.AssignOTelField(SemConv.ExceptionMessage, "connection refused");

// doc.Error.Message == "connection refused"
// doc.Attributes["exception.message"] == "connection refused"

Or set attributes directly on the dictionary:

var doc = new EcsDocument
{
    Message = "Request completed",
    Attributes = new MetadataDictionary
    {
        [SemConv.ExceptionMessage] = "connection refused",
        ["custom.field"] = "custom value",
    }
};

Serialization

The standard serializer writes ECS fields as ECS fields and the
Attributes dictionary as the attributes object:

var json = doc.Serialize();
// {
//   "@timestamp": "...",
//   "message": "Request completed",
//   "ecs.version": "...",
//   "attributes": {
//     "exception.message": "connection refused",
//     "custom.field": "custom value"
//   }
// }

Reading OTel format

EcsDocument.Deserialize() handles JSON that includes an attributes
object. OTel attribute names are automatically mapped to their ECS
equivalents:

var json = """{"attributes":{"exception.message":"timeout"}}""";
var doc = EcsDocument.Deserialize(json);

// doc.Error.Message == "timeout"
// doc.Attributes["exception.message"] == "timeout"

OTEL_RESOURCE_ATTRIBUTES integration

CreateNewWithDefaults processes OTEL_RESOURCE_ATTRIBUTES entries
through the mapping table. Equivalent mappings (e.g.
deployment.environment.name) set both the ECS field and Attributes.
Unknown attributes are stored in Attributes only.

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