Rollup merge of rust-lang#82421 - sunfishcode:wasi-metadata-size, r=alexcrichton

Add a `size()` function to WASI's `MetadataExt`.

WASI's `filestat` type includes a size field, so expose it in
`MetadataExt` via a `size()` function, similar to the corresponding Unix
function.

r? `````@alexcrichton`````

Author: Dylan-DPC

library/std/src/sys/wasi/ext/fs.rs

@@ -400,6 +400,8 @@ pub trait MetadataExt {
     fn ino(&self) -> u64;
     /// Returns the `st_nlink` field of the internal `filestat_t`
     fn nlink(&self) -> u64;
+    /// Returns the `st_size` field of the internal `filestat_t`
+    fn size(&self) -> u64;
     /// Returns the `st_atim` field of the internal `filestat_t`
     fn atim(&self) -> u64;
     /// Returns the `st_mtim` field of the internal `filestat_t`
@@ -418,6 +420,9 @@ impl MetadataExt for fs::Metadata {
     fn nlink(&self) -> u64 {
         self.as_inner().as_wasi().nlink
     }
+    fn size(&self) -> u64 {
+        self.as_inner().as_wasi().size
+    }
     fn atim(&self) -> u64 {
         self.as_inner().as_wasi().atim
     }