Fix labeled booleans and string

[Dexterp37]

Without this fix products using these types silently fail ping submission with the following error:

Error executing task: Error: Unable to create metric of unknown type labeled_boolean
    at createMetric (file:///mnt/d/Mozilla/rally/core-addon/node_modules/@mozilla/glean/dist/webext/core/metrics/utils.js:17:15)
    at createMetricsPayload (file:///mnt/d/Mozilla/rally/core-addon/node_modules/@mozilla/glean/dist/webext/core/metrics/database.js:30:28)
    at MetricsDatabase.getPingMetrics (file:///mnt/d/Mozilla/rally/core-addon/node_modules/@mozilla/glean/dist/webext/core/metrics/database.js:153:20)
    at processTicksAndRejections (internal/process/task_queues.js:93:5)
    at async collectPing (file:///mnt/d/Mozilla/rally/core-addon/node_modules/@mozilla/glean/dist/webext/core/pings/maker.js:98:25)
    at async collectAndStorePing (file:///mnt/d/Mozilla/rally/core-addon/node_modules/@mozilla/glean/dist/webext/core/pings/maker.js:122:30)
    at async file:///mnt/d/Mozilla/rally/core-addon/node_modules/@mozilla/glean/dist/webext/core/pings/ping_type.js:32:13
    at async Dispatcher.executeTask (file:///mnt/d/Mozilla/rally/core-addon/node_modules/@mozilla/glean/dist/webext/core/dispatcher.js:46:13)
    at async Dispatcher.execute (file:///mnt/d/Mozilla/rally/core-addon/node_modules/@mozilla/glean/dist/webext/core/dispatcher.js:66:21)

[brizental]

r+wc. I missed this step in our "Adding a new metric" guide. Would you mind including it? Follow up is fine too.

[Dexterp37]

r+wc. I missed this step in our "Adding a new metric" guide. Would you mind including it? Follow up is fine too.

I think we have that, but I just missed it last time