web: prometheus metrics support

[mcspr]

resolve #2323

Looks like:

relay0 0
relay1 0
relay2 0
relay3 0
temperature0 10.000
temperature1 9.000
humidity 35.000
pressure 997.000

I'll need to adjust sensor value printing, decimal places are hard-coded to 3 and need to be handled by the sensors module (somehow), as those methods aren't public right now.
HTTP auth won't work, as it seems. I can't find any config option to accept user:pass pair for the metrics target.

scrape_configs:
  - job_name: 'espurna'
    metrics_path: '/api/metrics'
    params:
      apikey: ['apikeyapikey']
    static_configs:
      - targets: ['espurna-blabla.lan:80']

[mcspr]

Using decimals from sensor / defaults:

relay0 0
temperature0 30.5
temperature1 29.5
humidity 56
pressure 1005.50

(i.e. for my DummySensor, temperature resolution is 0.1. Same for pressure, where it is 0.01. Humidity is a percentage, thus no decimals at all. Any specific sensor will override this in case value could fit more digits)

Path is /api/metrics?apikey=..., config example updated above.