server: provide statistics

[dneuge]

Clients may want to know availability of data file formats. When reading from storage, seen files could be counted and provided as a consolidated statistic in meta data (as file or header) regardless of requested format.

Example

{
  "statistics": {
    "availability_processed": {
        "total": {
                "first": "2020-12-29T00:00:06Z",
                "last": "2020-12-29T23:58:08Z",
        },
        "formats": {
            "legacy": {
                "files": 720,
                "first": "2020-12-29T00:00:06Z",
                "last": "2020-12-29T23:58:06Z",
                "fraction_of_max": 1.0
            },
            "json_v3": {
                "files": 256,
                "first": "2020-12-29T13:22:06Z",
                "last": "2020-12-29T23:58:08Z",
                "fraction_of_max": 0.356
            }
        }
    }
  }
}

This would indicate that legacy format is more available than json_v3 in the observed time period of a full day.