This was a problem for me when trying to get output from an SDS011 Nova
PM sensor. When parsed by the pometheus code it returns labels / metrics
like:
```# TYPE tasmota_sensors_pm2.5_ gauge
tasmota_sensors_pm2.5_{sensor="sds0x1"} 2.2```
The error from prometheus was:
```"append failed" err="invalid metric type \\"5_ gauge\\"```
Exports version, image type, and build timestamp.
It's common to export something like this to show version numbers across the fleet.
For example, the golang exporter exposes version:
```
go_info{version="go1.14.4"} 1
```
And prometheus exposes:
```
prometheus_build_info{branch="HEAD",goversion="go1.14.4",revision="eba3fdcbf0d378b66600281903e3aab515732b39",version="2.19.1"} 1
```