[cmd/mdatagen] Rename per-metric config types to include Metric infix
Rename generated types for per-metric reaggregation config to make the
category explicit and avoid future name collisions:
- `{MetricName}Config` -> `{MetricName}MetricConfig`
- `{MetricName}AttributeKey` -> `{MetricName}MetricAttributeKey`
For example, for the `system.cpu.frequency` metric:
- `SystemCPUFrequencyConfig` -> `SystemCPUFrequencyMetricConfig`
- `SystemCPUFrequencyAttributeKey` ->
`SystemCPUFrequencyMetricAttributeKey`
Without the infix, `{MetricName}Config` is ambiguous — it could be
confused with any other `Config` struct in the codebase. Adding `Metric`
makes the category explicit and consistent with the surrounding
generated types (`MetricsConfig`, `MetricsBuilderConfig`, etc.).
Also fix the `slices` import to only be emitted when there are actually
required attributes (which produce `slices.Contains` calls), rather than
whenever any aggregatable attributes exist.