The web3 usage metrics added in #9144 assumed that all web3 properties
were strings. When a `Symbol` property is accessed, our `inpage.js`
script crashes because the `Symbol` cannot be serialized correctly.
A check has been added for non-string property access. The metric event
in these cases is set to the string "typeof ", followed by the type of
the key. (e.g. `typeof symbol` for a `Symbol` property).
Fixes#9234
