add build metrics, to gather ci stats from x.py

This tool will generate a JSON file with statistics about each
individual step to disk. It will be used in rust-lang/rust's CI to
replace the mix of scripts and log scraping we currently have to gather
this data.

src/bootstrap/lib.rs

@@ -149,6 +149,9 @@ mod tool;
 mod toolstate;
 pub mod util;
 
+#[cfg(feature = "build-metrics")]
+mod metrics;
+
 #[cfg(windows)]
 mod job;
 
@@ -311,6 +314,9 @@ pub struct Build {
     prerelease_version: Cell<Option<u32>>,
     tool_artifacts:
         RefCell<HashMap<TargetSelection, HashMap<String, (&'static str, PathBuf, Vec<String>)>>>,
+
+    #[cfg(feature = "build-metrics")]
+    metrics: metrics::BuildMetrics,
 }
 
 #[derive(Debug)]
@@ -500,6 +506,9 @@ impl Build {
             delayed_failures: RefCell::new(Vec::new()),
             prerelease_version: Cell::new(None),
             tool_artifacts: Default::default(),
+
+            #[cfg(feature = "build-metrics")]
+            metrics: metrics::BuildMetrics::init(),
         };
 
         build.verbose("finding compilers");
@@ -692,6 +701,9 @@ impl Build {
             }
             process::exit(1);
         }
+
+        #[cfg(feature = "build-metrics")]
+        self.metrics.persist(self);
     }
 
     /// Clear out `dir` if `input` is newer.