it will listen on the configured mqtt server at `$MQTT_TOPIC` for metrics in json_format.
The payload needs to be an object containing only the following keys:
-`name` (string): the name of the metric
-`value` (number): the value of the metric
-`labels` (object, optional): an object containing the labels for the metric (key-value pairs of strings)
According to prometheus conventions, the metric name and the label names need to be a lower or upper case letter or a underscore followed by all case leters, numbers or underscores.
These are all valid payloads:
```json
{
"name": "temperature",
"value": 30.7,
"labels": {
"unit": "°C",
"location": "Werkstatt"
}
}
```
```json
{
"name": "temperature",
"value": 30.7,
"labels": {}
}
```
```json
{
"name": "temperature",
"value": 30.7
}
```
The service will expose the metrics on the `/metrics` endpoint.
All metrics will be presented for about 5 minutes, and exported with their eventtime as a timestamp.
Metric types are not currently implemented, all metrics are exported without a type.
This might change when prometheus supports it.
The service will report errors in received messages to STDERR and to $MQTT_TOPIC/error.
Possible errors are:
- invalid json
- missing `name` or `value` key
- invalid `name` (prometheus conventions)
- invalid `value` (not a number)
- invalid `labels` (not an object)
- invalid label names (not a sting or not a valid prometheus label name)
- invalid label values (not a string)
- additional information in the payload (not one of `name`, `value`, `labels`)
The error message describes the error type and repeats the payload that caused the error.
There is a comment block on top if the metric that shows some metadata for human consumption.
mem usage and mem usage peak is according to [memory_get_usage()](https://www.php.net/manual/en/function.memory-get-usage.php) and [memory_get_peak_usage()](https://www.php.net/manual/en/function.memory-get-peak-usage.php) respectively.
in both cases it gives 2 values, the first is the acutally used value, the second is tha allocated.
A new export is written either
- once oper second if new events are incoming.
- every 15 seconds if no new events are incoming, to update stats and remove stale entries.
There is a 60 second period on startup where metrics are received but not exported, in case the service crashes and there are previous exported metrics that are not polled by prometheus yet.
