Add OpenTelemetry Logs Ingestion Support

[a-khaledf]

Summary

This PR adds support for ingesting logs via the OpenTelemetry /v1/logs endpoint, controllable via the AXIOM_OTEL_ENABLED environment variable or the SetOtelEnabled client option.

Changes

New Features

• IngestOtel function: New method on Client and DatasetsService that sends events to the /v1/logs endpoint in proper OTLP (OpenTelemetry Protocol) JSON format
• SetOtelEnabled client option: Programmatically enable OTel-based ingestion
• AXIOM_OTEL_ENABLED environment variable: Set to "true" or "1" to enable OTel ingestion globally
• Automatic routing: When OTel is enabled, IngestEvents automatically delegates to IngestOtel

Implementation Details

• Events are converted to OTLP log format using go.opentelemetry.io/collector/pdata/plog
• Requests are sent to /v1/logs with X-Axiom-Dataset header specifying the target dataset
• Content-Type is application/json with zstd compression
• Event fields are mapped to OTLP log record attributes
• The _time field is used for the OTLP timestamp if present

Files Changed

• internal/config/config.go - Added otelEnabled field and getter/setter
• internal/config/option.go - Added SetOtelEnabled config option
• axiom/client_options.go - Added SetOtelEnabled client option
• axiom/client.go - Added IngestOtel alias and mediaTypeOTelLogs constant
• axiom/datasets.go - Added IngestOtel function and eventsToOTLP helper
• axiom/datasets_test.go - Added tests for IngestOtel and OTel-enabled IngestEvents
• internal/config/config_test.go - Added tests for OTel config options