feat: loki-logger support for structured metadata

[marek-obuchowicz]

Description

As recommended in loki documentation, labels should be used for fields with limited cardinality - a good example would be http request method, which can have one of few values. Labels should not be used for indexing fields with high value cardinality, like IP address or request id. For this case, loki has a feature called "structured metadata" - it can and should be used to index searchable fields.

At the moment, loki-logger plugin allows defining loki labels, but does not provide an option to specify structured metadata. This makes the plugin hardly usable in real-world, high-traffic scenarios where we need to index fields with high cardinality.

[Baoyuantop]

Thanks for the request. From the current APISIX docs and loki-logger implementation, the plugin supports log_labels and log_format, but it does not currently expose a structured metadata configuration or add structured metadata to the Loki push payload.

This looks like a valid enhancement for the loki-logger plugin rather than a bug in the existing documented behavior. Could you share the expected APISIX configuration shape and the exact payload mapping you would like to see? A PR would need to add the new optional schema field, keep the existing log_labels/log_format behavior backward compatible, and include tests and docs.