Heroku Fir support

We've added Heroku Fir support.

By pointing Heroku Fir's telemetry drain to our collector we report the following:

• Host metrics: CPU usage, memory usage and free memory.
• Metrics from add-ons, such as PostgreSQL and Redis. These will be visible in automatically added dashboards.
• Logs from the Heroku router and API.

For more information, see the Heroku Fir sections in our Heroku documentation.