Telegraf alt_baro datatype conversion (#144)

The alt_baro readsb metric produces either a numeric value or the string
"ground".

Prometheus doesn't support strings as metrics and will ignore strings.
Therefore, there is no problem with Prometheus having a forced data
type.

In the case of InfluxDB 2, the first value of the key submitted will
define the data type of that field. When "ground" is the first value,
Influx will only populate the field when the value is "ground" and fire
an error that Influx couldn't convert float to string. This will cause
no altitude metrics to be collected.

To keep InfluxDB metrics in line with Prometheus, the data type must be
forced as a float.

This does not affect Prometheus since it's dumping strings anyway, and
it brings Influx data on par with the Prometheus data.

Further work needs to be done to collect the "ground" value to retain
the data, but this will fix the issue of starting a new bucket and not
gathering altitude metrics.

rootfs/etc/telegraf/telegraf.d/readsb_aircraft_json.conf

@@ -12,3 +12,6 @@
 [[processors.strings]]
   [[processors.strings.trim]]
     tag = "flight"
+[[processors.converter]]
+  [processors.converter.fields]
+    float = ["alt_baro"]