Add a customizable title to Metrics reporting

[kripken]

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.

This is now trivial to do, after #6687

cc @gkdn , I believe you asked for something like this in the past.