Add N1QL query metrics option and server results

It's not currently possible to set the metrics option directly in a N1QL request, like other SDKs. Instead, it must be done through the `set_option` function and it's results are added to the result's meta dictionary.

Example of requesting via `set_option`

query = N1QLQuery('SELECT 1 from `default`')

query.set_option('metrics', True)

result = b.n1ql_query(query)

# have to process all rows first

for row in result:

    print row

# metrics is an entry in result.meta

print result.meta