Add -Zerror-metrics=PATH to save diagnostic metadata to disk

compiler/rustc_session/src/options.rs

@@ -1827,6 +1827,8 @@ options! {
         the same values as the target option of the same name"),
     meta_stats: bool = (false, parse_bool, [UNTRACKED],
         "gather metadata statistics (default: no)"),
+    metrics_dir: Option<PathBuf> = (None, parse_opt_pathbuf, [UNTRACKED],
+        "stores metrics about the errors being emitted by rustc to disk"),
     mir_emit_retag: bool = (false, parse_bool, [TRACKED],
         "emit Retagging MIR statements, interpreted e.g., by miri; implies -Zmir-opt-level=0 \
         (default: no)"),