cc @ampproject/wg-analytics

In the following example, nested macro doesn't work properly

$REPLACE(CLIENT_ID(navigationStart)test, 'AD_', '')"
resolves to client_id, note that test is dropped

$REPLACE(testCLIENT_ID(navigationStart), 'AD_', '')"
resolves to test, note that client_id is dropped

$REPLACE(NAV_TIMING(navigationStart, loadEventStart), 'AD_', '')"
resolves to empty string, note that the nav_timing number is dropped.

I believe there are two issues here

1. Nested macro doesn't handle number properly
2. Nested macro doesn't handle concat string properly