Support limiting tracing helper's scope

[st0012]

With a Block

print_calls_in_detail(ActionDispatch::Http::URL) do
  ActionDispatch::Http::URL.url_for(host: "www.ror.co.uk", subdomain: "api", tld_length: 2)
end

# ignore what happens outside the block

With stop_tracing

print_calls_in_detail(ActionDispatch::Http::URL)
result = ActionDispatch::Http::URL.url_for(host: "www.ror.co.uk", subdomain: "api", tld_length: 2)

stop_tracing # ignore what happens after this line