Add repr to FilterX Otel objects

[bshifter]

Axorouter encountered an error:
filterx: unable to repr; from='otel_kvlist', to='string'.

Upon investigating the issue, I found relevant hints in Axorouter’s configuration. The configuration suggests that OTel objects still require a repr method, but currently, a JSON-based workaround is used:

      filterx {
        # Fallback if map-to-text is not defined for the appliance.
        # TODO: OTel based types do not support string() cast, so we fallback for format_json().
        #       Remove that when OTel types start supporting string() cast.
        log.body = string(log.body) ?? format_json(log.body);

To address this, I implemented repr methods for all OTel-related FilterX objects:

• array
• kvlist
• logrecord
• resource
• scope

Additionally, I provided light tests to validate repr methods using forced string typecasting.

[OverOrion]

MessageToJsonString returns a Status. It might seem overly cautious, but could you please check it's value to make sure the output buffer's contents is valid?