Add support for tracking custom metrics

Builds can now emit metrics that Hydra will store in its database and
render as time series via flot charts. Typical applications are to
keep track of performance indicators, coverage percentages, artifact
sizes, and so on.

For example, a coverage build can emit the coverage percentage as
follows:

  echo "lineCoverage $pct %" > $out/nix-support/hydra-metrics

Graphs of all metrics for a job can be seen at

  http://.../job/<project>/<jobset>/<job>#tabs-charts

Specific metrics are also visible at

  http://.../job/<project>/<jobset>/<job>/metric/<metric>

The latter URL also allows getting the data in JSON format (e.g. via
"curl -H 'Accept: application/json'").

src/lib/Hydra/Controller/Job.pm

@@ -77,6 +77,9 @@ sub overview : Chained('job') PathPart('') Args(0) {
         , jobset => $c->stash->{jobset}->name
         , job => $c->stash->{job}->name
         })->count == 1 if $c->user_exists;
+
+    $c->stash->{metrics} = [ $job->buildmetrics->search(
+        { }, { select => ["name"], distinct => 1, order_by => "timestamp desc",  }) ];
 }
 
 
@@ -110,6 +113,20 @@ sub output_sizes : Chained('job') PathPart('output-sizes') Args(0) {
 }
 
 
+sub metric : Chained('job') PathPart('metric') Args(1) {
+    my ($self, $c, $metricName) = @_;
+
+    $c->stash->{template} = 'metric.tt';
+    $c->stash->{metricName} = $metricName;
+
+    my @res = $c->stash->{job}->buildmetrics->search(
+        { name => $metricName },
+        { order_by => "timestamp", columns => [ "build", "name", "timestamp", "value", "unit" ] });
+
+    $self->status_ok($c, entity => [ map { { id => $_->get_column("build"), timestamp => $_ ->timestamp, value => $_->value, unit => $_->unit } } @res ]);
+}
+
+
 # Hydra::Base::Controller::ListBuilds needs this.
 sub get_builds : Chained('job') PathPart('') CaptureArgs(0) {
     my ($self, $c) = @_;