Add json output to -Zdump-mono-stats

This allows analyzing the output programatically; for example, finding
the item with the highest `total_estimate`.

I also took the liberty of adding `untracked` tests to `rustc_session` and documentation to the unstable book for `dump-mono-items`.

compiler/rustc_session/src/config.rs

@@ -2975,3 +2975,21 @@ pub enum ProcMacroExecutionStrategy {
     /// Run the proc-macro code on a different thread.
     CrossThread,
 }
+
+/// Which format to use for `-Z dump-mono-stats`
+#[derive(Clone, Copy, PartialEq, Hash, Debug)]
+pub enum DumpMonoStatsFormat {
+    /// Pretty-print a markdown table
+    Markdown,
+    /// Emit structured JSON
+    Json,
+}
+
+impl DumpMonoStatsFormat {
+    pub fn extension(self) -> &'static str {
+        match self {
+            Self::Markdown => "md",
+            Self::Json => "json",
+        }
+    }
+}

compiler/rustc_session/src/options.rs

@@ -377,6 +377,7 @@ mod desc {
     pub const parse_linker_flavor: &str = ::rustc_target::spec::LinkerFlavorCli::one_of();
     pub const parse_optimization_fuel: &str = "crate=integer";
     pub const parse_mir_spanview: &str = "`statement` (default), `terminator`, or `block`";
+    pub const parse_dump_mono_stats: &str = "`markdown` (default) or `json`";
     pub const parse_instrument_coverage: &str =
         "`all` (default), `except-unused-generics`, `except-unused-functions`, or `off`";
     pub const parse_unpretty: &str = "`string` or `string=string`";
@@ -820,6 +821,21 @@ mod parse {
         true
     }
 
+    pub(crate) fn parse_dump_mono_stats(slot: &mut DumpMonoStatsFormat, v: Option<&str>) -> bool {
+        match v {
+            None => true,
+            Some("json") => {
+                *slot = DumpMonoStatsFormat::Json;
+                true
+            }
+            Some("markdown") => {
+                *slot = DumpMonoStatsFormat::Markdown;
+                true
+            }
+            Some(_) => false,
+        }
+    }
+
     pub(crate) fn parse_instrument_coverage(
         slot: &mut Option<InstrumentCoverage>,
         v: Option<&str>,
@@ -1295,7 +1311,9 @@ options! {
         an additional `.html` file showing the computed coverage spans."),
     dump_mono_stats: SwitchWithOptPath = (SwitchWithOptPath::Disabled,
         parse_switch_with_opt_path, [UNTRACKED],
-        "output statistics about monomorphization collection (format: markdown)"),
+        "output statistics about monomorphization collection"),
+    dump_mono_stats_format: DumpMonoStatsFormat = (DumpMonoStatsFormat::Markdown, parse_dump_mono_stats, [UNTRACKED],
+        "the format to use for -Z dump-mono-stats (`markdown` (default) or `json`)"),
     dwarf_version: Option<u32> = (None, parse_opt_number, [TRACKED],
         "version of DWARF debug information to emit (default: 2 or 4, depending on platform)"),
     dylib_lto: bool = (false, parse_bool, [UNTRACKED],