input: decoder: trace input fifo buffer level

This MR adds support for unsigned integers in the tracer, and use this support to trace the decoder fifo level.

The decoder fifo level metric allows interpreting the situation that led to the fifo from the decoder to be flushed (ie. the fifo reached 400MiB of undecoded data stored). The metrics generates a lot of points though, so be sure to aggregate when displaying it.

Example of query in flux language:

    from(bucket: v.defaultBucket)
    |> range(start: v.timeRangeStart, stop:v.timeRangeStop)
    |> filter(fn: (r) =>
      r._measurement == "measurement" and
      r._field == "fifo_size"
    )
    |> aggregateWindow(every: 1s, fn: max)

Unaggregated:

Aggregated: