Add a customizable title to Metrics reporting (#6792)

Before the PR:

$ bin/wasm-opt test/hello_world.wat --metrics
total
 [exports]      : 1       
 [funcs]        : 1       
 [globals]      : 0       
 [imports]      : 0       
 [memories]     : 1       
 [memory-data]  : 0       
 [tables]       : 0       
 [tags]         : 0       
 [total]        : 3       
 [vars]         : 0       
 Binary         : 1       
 LocalGet       : 2       

After the PR:

$ bin/wasm-opt test/hello_world.wat --metrics
Metrics
total
 [exports]      : 1       
 [funcs]        : 1       
...

Note the "Metrics" addition at the top. And the title can be customized:

$ bin/wasm-opt test/hello_world.wat --metrics=text
Metrics: text
total
 [exports]      : 1       
 [funcs]        : 1       

The custom title can be helpful when multiple invocations of metrics are used
at once, e.g. --metrics=before -O3 --metrics=after.

2 files changed, 10 insertions, 1 deletions

diff --git a/src/passes/Metrics.cpp b/src/passes/Metrics.cpp
index 1778eb9bd..186d7e695 100644
--- a/src/passes/Metrics.cpp
+++ b/src/passes/Metrics.cpp
@@ -45,6 +45,13 @@ struct Metrics
   }
 
   void doWalkModule(Module* module) {
+    std::string title = getArgumentOrDefault("metrics", "");
+    std::cout << "Metrics";
+    if (!title.empty()) {
+      std::cout << ": " << title;
+    }
+    std::cout << '\n';
+
     ImportInfo imports(*module);
 
     // global things
diff --git a/src/passes/pass.cpp b/src/passes/pass.cpp
index ec6077941..3f84ee604 100644
--- a/src/passes/pass.cpp
+++ b/src/passes/pass.cpp
@@ -275,7 +275,9 @@ void PassRegistry::registerPasses() {
                createMergeSimilarFunctionsPass);
   registerPass(
     "merge-locals", "merges locals when beneficial", createMergeLocalsPass);
-  registerPass("metrics", "reports metrics", createMetricsPass);
+  registerPass("metrics",
+               "reports metrics (with an optional title, --metrics[=TITLE])",
+               createMetricsPass);
   registerPass("minify-imports",
                "minifies import names (only those, and not export names), and "
                "emits a mapping to the minified ones",