Prior to this change, the custom queries were restricted to counters and
gauges.

This change introduces a new ColumnUsage, namely HISTOGRAM, that expects
the column to contain an array of upper inclusive bounds for each
observation bucket in the emitted metric.  It also expects three more
columns to be present with the suffixes:
- `_bucket`, containing an array of cumulative counters for the
  observation buckets;
- `_sum`, the total sum of all observed values; and
- `_count`, the count of events that have been observed.

A flag has been added to the MetricMap struct to easily identify metrics
that should emit a histogram and the construction of a histogram metric
is aided by the pg.Array function and a new helper dbToUint64 function.

Finally, and example of usage is given in queries.yaml.

fixes #402

Signed-off-by: Corin Lawson <corin@responsight.com>

Prior to this change, the histogram support was untested.

This change introduces a new integration test that reads a user query
containing a number of histogram metrics.  Also, additional checks have
been added to TestBooleanConversionToValueAndString to test dbToUint64.

Signed-off-by: Corin Lawson <corin@responsight.com>

Prior to this change, the test-smoke script required the p2 binary to
augment the Dockerfile under docker-postgres-replication.  I do not have
that binary on my system, nor was I able to find an installer for it.
The closest tool I found was p2cli, but even that did not work as
expected.

This change removes the Dockerfile.p2 file in favour of using Docker's
builtin build args mechanism to augment the docker build.

Signed-off-by: Corin Lawson <corin@responsight.com>

Not all systems have /bin/bash available (mine among them).

This change makes use of /usr/bin/env in place of /bin/bash.