Track number of strings, words and characters sent for translation

This is related to https://zanata.atlassian.net/browse/ZNTAMT-32#icft=ZNTAMT-32 - although we no longer create a TextFlow for every MT request, we still need to track the cost of translations for every request, including the duplicate requests.

We should count the number of strings, words and characters sent for translation. Word counts should be calculated according to GMX-V 2.0 (as used in Zanata Platform's HTextFlow class via Okapi Word Count).