Add structured metadata support for Loki output plugin"

[error9098x]

Signed-off-by: ProCodec 43810146+error9098x@users.noreply.github.com

What this PR does / why we need it:

Adds support for structured metadata in the Loki output plugin. This allows attaching high-cardinality data (like pod names, trace IDs, thread IDs) to log lines without using them as labels, following Loki's recommended best practices for Loki 3.0+.

The implementation adds:

• structuredMetadata field to set key-value pairs as structured metadata
• structuredMetadataKeys field to reference record keys for structured metadata
• Processing logic in the Params() method to generate the proper Fluent Bit configuration
• Test cases to verify the functionality

Which issue(s) this PR fixes:

Fixes #1551

Does this PR introduced a user-facing change?

Added support for structured metadata in Loki output plugin. Users can now configure high-cardinality data to be sent as structured metadata instead of labels.

Note: This feature requires Loki 3.0+ with schema v13 or higher, TSDB storage, and allow_structured_metadata: true in Loki configuration.

Additional documentation, usage docs, etc.:

Example usage:

apiVersion: fluentbit.fluent.io/v1alpha2
kind: ClusterOutput
metadata:
name: loki-output
spec:
match: kube.*
loki:
host: loki-gateway
port: 3100
labels:
- job=fluentbit
- environment=production
structuredMetadata:
pod: "${record['kubernetes']['pod_name']}"
      container: "${record['kubernetes']['container_name']}"
trace_id: "\${record['trace_id']}"
structuredMetadataKeys:
- level
- caller

For more info on structured metadata: https://grafana.com/docs/loki/latest/get-started/labels/structured-metadata/