Emit current SDR gain in the local receiver stats

[mutability]

Always emit this, even if adaptive gain is disabled.

For backwards compatibility the gain is still emitted in the old stats location as well if adaptive gain is enabled.

sample output:

{
  "latest": {
    "start": 1665032717.8,
    "end": 1665032777,
    "local": {      
      "samples_processed": 142213120,
      "samples_dropped": 0,
      "modeac": 0,
      "modes": 875641,
      "bad": 3471481,
      "unknown_icao": 369459,
      "accepted": [
        0
      ],
      "noise": -11.1,
      "strong_signals": 0,
      "gain_db": 49.6              <-------
    },
...

[mutability]

@wiedehopf does this fit OK with graphs1090?

[wiedehopf]

Made a change to graphs1090, it should grab either version.