Add support for dynamic object key extraction in JSON Exporter

[silveraignacio]

This enhancement allows extracting dynamic JSON object keys as Prometheus labels using the special JSONPath syntax {__name__}.

Key Features:

• Extract dynamic object keys as label values using {__name__} syntax
• Maintain full backward compatibility with existing configurations
• Support for mixed label types (dynamic keys + static values + regular JSONPath)
• Enhanced object scraping with dynamic label extraction

Use Cases:

• Cloud provider monitoring with dynamic instance/account names
• Service discovery endpoints with generated names
• Infrastructure monitoring with environment-specific identifiers
• API gateway monitoring with dynamic endpoint names

Implementation:

• Added extractDynamicLabels() function for enhanced label extraction
• Added extractDynamicValue() function for dynamic object value extraction
• Extended object scraping logic to support dynamic key extraction
• Added comprehensive test coverage for new functionality
• Included test data and configuration files for validation

Files Added/Modified:

• exporter/collector.go: Core implementation
• exporter/util_test.go: Unit tests
• README.md: Updated documentation
• test/local-test-. and test/multi-provider-test-.: Test configurations and data

Example

{"providers": [{"aws-prod-123": {"ok": true}}, {"gcp-dev-456": {"ok": false}}]}

Config

    - name: provider
      type: object
      help: Provider account status (1=ok, 0=failed)
      path: '{ .providers[*] }'
      labels:
        provider: '{__name__}'  # Extract dynamic object key as label
      values:
        status: '{.ok}'  # Boolean converted to 1/0

Generates:

provider_status{provider_name="aws-prod-123"} 1
provider_status{provider_name="gcp-dev-456"} 0