opt-dist: explicitly pass --set build.metrics=true while running build

at first it's not obvious that build.metrics should be enabled to use
opt-dist which will lead to confusion. probably it's better to add a
good error message which instructs to set this option manually

Author: ognevny

src/tools/opt-dist/src/exec.rs

@@ -111,6 +111,8 @@ impl Bootstrap {
             "--stage",
             "2",
             "library/std",
+            "--set",
+            "build.metrics=true",
         ])
         .env("RUST_BACKTRACE", "full");
         Self { cmd, metrics_path }