Support non-string bodies on OpenTelemetry logs

[KodrAus]

The OpenTelemetry spec for log bodies suggests they should be string values, but may be structured data. Fluent Bit's OpenTelemetry output is an example of a source that uses non-string bodies. See also fluent/fluent-bit#7649.

Seq's /ingest/otlp/v1/logs endpoint currently only maps the body if it's a string value, meaning logs ingested directly from Fluent Bit come through as empty.

One workaround is to put the OpenTelemetry Collector between Fluent Bit and Seq, since the collector has a specific input for Fluent Bit that unpacks its values into attributes.

Another workaround is to use Fluent Bit's generic HTTP output and send logs as CLEF directly to Seq as described in this blog post.