Transformation of Meter to Influx line protocol has always 'unknown' metric_type

Fork from issue #1296.

Transformation of Meter to Influx line protocol adds Tag metric_type with hardcoded value unknown. I think it should respect the registered type from Meter.getId().getType().

        Stream.Builder<Field> fields = Stream.builder();
        for (Measurement measurement : m.measure()) {
            String fieldKey = measurement.getStatistic().toString()
                    .replaceAll("(.)(\\p{Upper})", "$1_$2").toLowerCase();
            fields.add(new Field(fieldKey, measurement.getValue()));
        }

        return Stream.of(influxLineProtocol(m.getId(), "unknown", fields.build()));

I’d propose following:

1. use Meter.getId().getType().toLowerCase() directly in influxLineProtocol(), drop the String metricType parameter

or in case this would cause an internal API change

1. use Meter.getId().getType().toLowerCase() in writeMeter() method instead of unknown

[izeye]

I created #1446 to try to resolve this as @zeratul021 suggested.

[zeratul021]

I suppose there are cases for those types with semantics that differ from Micrometer’s implementation. So this change seems reasonable to me.

Exactly our use-cases.

Meter type is an enum so if I’m forced to choose between other and anything more specific, I’ll lean towards the latter.

Though changing the value of the metric_type tag in a point release for custom meters would be a breaking change for people relying on the current value.

You are right. I was thinking of that too. Maybe we could add a new field metricType to Meter (DefaultMeter) that would be translated to metric_type with default value set to current unknown. That way it would be backwards compatible and properly fixed in 1.2.

Anyway, no rush to get that, just suggestion.