Update default dashboard for ext-kentik_ping to use more relevant labels

[throck]

this is a dashboard update only

the default ping dashboard was incomplete in that it didn't show all expected metrics for a ping result; simple re-work here that uses more appropriate labels (latency/loss/jitter) and corrects the incorrrect 'failed pings' chart

Checklist

• [X] I've read the guidelines and understand the acceptance criteria.

[CLAassistant]

All committers have signed the CLA.

[lujop]

Check automatic validations and remove the permissons that we don't put in the definition files. IIRC accountIds isn't supported and you need to use accountId: 0 like you had before

[otaviocarvalho]

As @lujop mentioned, it is failing a few dashboard validations. Please reach out if you need support with it.