db: reformat Metrics.String() output

[jbowens]

The Metrics.String() output is often used in debugging storage issues. As we've accumulated more statistics that are not per-level, each has been appended as an independent line with descriptive annotations like (size == estimated-debt, score = in-progress-bytes, in = num-in-progress).

With a little thought and elbow grease, I think these could be restructured to be a little more legible and avoid the excess vertical height.

//   __level_____count____size___score______in__ingest(sz_cnt)____move(sz_cnt)___write(sz_cnt)____read___w-amp
//       WAL         1    27 B       -    48 B       -       -       -       -   108 B       -       -     2.2
//         0         2   1.6 K    0.50    81 B   825 B       1     0 B       0   2.4 K       3     0 B    30.6
//         1         0     0 B    0.00     0 B     0 B       0     0 B       0     0 B       0     0 B     0.0
//         2         0     0 B    0.00     0 B     0 B       0     0 B       0     0 B       0     0 B     0.0
//         3         0     0 B    0.00     0 B     0 B       0     0 B       0     0 B       0     0 B     0.0
//         4         0     0 B    0.00     0 B     0 B       0     0 B       0     0 B       0     0 B     0.0
//         5         0     0 B    0.00     0 B     0 B       0     0 B       0     0 B       0     0 B     0.0
//         6         1   825 B    0.00   1.6 K     0 B       0     0 B       0   825 B       1   1.6 K     0.5
//     total         3   2.4 K       -   933 B   825 B       1     0 B       0   4.1 K       4   1.6 K     4.5
//     flush         3
//   compact         1   1.6 K     0 B       1          (size == estimated-debt, score = in-progress-bytes, in = num-in-progress)
//     ctype         0       0       0       0       0  (default, delete, elision, move, read)
//    memtbl         1   4.0 M
//   zmemtbl         0     0 B
//      ztbl         0     0 B
//    bcache         4   752 B    7.7%  (score == hit-rate)
//    tcache         0     0 B    0.0%  (score == hit-rate)
//    titers         0
//    filter         -       -    0.0%  (score == utility)