Internal metrics collection

[jimaek]

For tasks like https://github.com/jsdelivr/globalping/issues/37 and many others we will need to know more about the probes.

Here is a list of data we would probably need to collect to build the next features:

• Accepted tests, in progress tests, finished tests, failed tests. Accepted/in progress probably just real-time value, while the rest are timeseries. Maybe even total and per type as well?
• CPU load and CPU cores available
• Uptime
• Other?

We should probably store them for up to 7 days in a timeseries DB. But note that whatever DB we choose it will need it to scale as in the future we will also support scheduled tests, like pinging the same target every minute and building a chart of performance over time per region.