remove formatting from ResettingTimer metrics if requested in raw format

[gravityblast]

metrics.ResettingTimer are always formatted as strings even when debug_metrics is called passing true, which is used to get raw metrics.
In this PR I removed the formatting of metrics.ResettingTimer metrics if raw is true.

here an example, we can look at trie.memcache.flush.time metrics, that have the final s:

> debug.metrics(true).trie.memcache.flush
{
  nodes: {
    AvgRate01Min: 0,
    AvgRate05Min: 0,
    AvgRate15Min: 0,
    MeanRate: 0,
    Overall: 0
  },
  size: {
    AvgRate01Min: 0,
    AvgRate05Min: 0,
    AvgRate15Min: 0,
    MeanRate: 0,
    Overall: 0
  },
  time: {
    Mean: "0s",
    Measurements: 0,
    Percentiles: {
      20: "0s",
      5: "0s",
      50: "0s",
      80: "0s",
      95: "0s"
    }
  }
}

on this branch:

> debug.metrics(true).trie.memcache.flush
{
  nodes: {
    AvgRate01Min: 0,
    AvgRate05Min: 0,
    AvgRate15Min: 0,
    MeanRate: 0,
    Overall: 0
  },
  size: {
    AvgRate01Min: 0,
    AvgRate05Min: 0,
    AvgRate15Min: 0,
    MeanRate: 0,
    Overall: 0
  },
  time: {
    Mean: 0,
    Measurements: 0,
    Percentiles: {
      20: 0,
      5: 0,
      50: 0,
      80: 0,
      95: 0
    }
  }
}

[karalabe]

LGTM, thanks for this!