metricsd: Allows introspecting the metrics.

This CL adds a dump command to metrics_client that will dump the
histograms, aggregated by metricsd since it started, in a human readable
format.
This is useful for developers to ensure that their code uses libmetrics
correctly and the reported values are correct.

Sample output (redacted to fit the commit message):
$ metrics_client -d
Histogram: hello recorded 5 samples, average = 54.8 (flags = 0x1)
0   ...
9   --------O                 (1 = 20.0%) {0.0%}
16  O                         (0 = 0.0%) {20.0%}
29  --------O                 (1 = 20.0%) {20.0%}
54  ------------------------O (3 = 60.0%) {40.0%}
100 O                         (0 = 0.0%) {100.0%}
$

Bug: 25817310
Test: * Send a histogram with metrics_client.
      * `metrics_client -d` shows it.

Change-Id: Id186dc5463403ca9181ee9eef8f46b5e809b8714