feat: expose registered metrics

[ivankatliarchuk]

What This PR Does

Adds programmatic access to registered metric descriptors, eliminating the need for unsafe reflection to introspect metrics.

New APIs:

1. Desc getter methods:

   • Name() string - Returns metric name
   • Help() string - Returns help text
   • ConstLabels() Labels - Returns constant labels
   • VariableLabels() []string - Returns variable label names

2. Registry introspection:

   • Descriptors() []*Desc - Returns all registered metric descriptors

Benefits

• Generate documentation from running services
• Validate metric configurations programmatically
• Build metric discovery and debugging tools
• Replace unsafe reflection hacks (external-dns currently needs ~500 lines of workarounds)

Testing

• Runnable example in examples/introspection/

Fixes #1801

Here's a practical example of how this simplifies real-world code.

  **Before (external-dns approach):**
  ```go
  // UNSAFE: Uses reflection to access private registry fields
  func extractMetrics(reg *prometheus.Registry) (map[string]MetricInfo, error) {
      // Get private dimHashesByName field using unsafe pointer manipulation
      registryValue := reflect.ValueOf(reg).Elem()
      dimHashesField := registryValue.FieldByName("dimHashesByName")
      dimHashesField = reflect.NewAt(
          dimHashesField.Type(),
          unsafe.Pointer(dimHashesField.UnsafeAddr()),
      ).Elem()

      // Extract and process descriptors (50+ more lines)
      // ...
  }

  After (with this PR):
  func extractMetrics(reg *prometheus.Registry) []MetricInfo {
      descs := reg.Descriptors()
      metrics := make([]MetricInfo, len(descs))

      for i, desc := range descs {
          metrics[i] = MetricInfo{
              Name:           desc.Name(),
              Help:           desc.Help(),
              ConstLabels:    desc.ConstLabels(),
              VariableLabels: desc.VariableLabels(),
          }
      }

      return metrics
  }

Clean, safe, and maintainable