Add use super::* to define_stats! generated module for type hygiene

Summary:
The `define_stats!` macro generates a `pub(crate) mod STATS { ... }`
block. Type parameters in dynamic stat definitions (e.g. `region: String`)
are forwarded as `tt` token trees and only parsed as types inside this
generated module. This means custom types from the caller's scope were not
resolvable — only primitive types and `&'static str` worked.

Adding `use super::*;` to the generated module makes all of the caller's
imports visible, so types like `dba_common_types::Region` can be used
directly in stat definitions:

```rust
use dba_common_types::Region;

define_stats! {
    prefix = "my.prefix";
    my_stat: dynamic_timeseries("{}", (region: Region); Count),
}
```

All three resolution paths now work:
- Direct name: `region: Region` (via `use super::*`)
- Explicit super: `region: super::Region`
- Fully qualified: `region: ::dba_common_types::Region`

This is backward-compatible — existing code using `String`, `u64`,
`&'static str` etc. continues to work unchanged.

Reviewed By: dtolnay

Differential Revision: D99868296

fbshipit-source-id: 30a75bb943d86751c6a28a79a85d50885921abaa

Author: macisamuele

common/rust/shed/stats/src/macros.rs

@@ -202,6 +202,10 @@ macro_rules! define_stats {
         #[allow(non_snake_case, non_upper_case_globals, unused_imports, clippy::redundant_pub_crate)]
         pub(crate) mod STATS {
             use $crate::macros::common_macro_prelude::*;
+            // Import caller's symbols so that custom types (e.g. newtypes implementing
+            // Display) can be used as dynamic stat key parameters without requiring
+            // `super::` or fully qualified paths.
+            use super::*;
 
             static STATS_MAP: LazyLock<Arc<ThreadMap<BoxStatsManager>>> = LazyLock::new(|| create_map());
             static STATS_MANAGER: LazyLock<BoxStatsManager> = LazyLock::new(|| create_stats_manager());