- magnitudes vector grows by calling copy ctor, ensure filter never gets deleted
- ensure we can't use magnitude member instead of ctor arg
- more magnitude references instead of using index access
- additional checks for isnan, fix report never triggering
- drop event callback. at least for EventSensor, we already have
  report with min / max value change triggers

- (experimental) provide generic way to read magnitude values
- expose /api/metrics with values formatted specifically for prometheus, with relay and sensor data
- small tweaks to sensor init

Example config:
```
scrape_configs:
  - job_name: 'espurna'
    metrics_path: '/api/metrics'
    params:
      apikey: ['apikeyapikey']
    static_configs:
      - targets: ['espurna-blabla.lan:80']
```
Where ESPurna side has
```
apiKey => "apikeyapikey"
apiEnabled => "1"
```