Added the InfluxDBNotification plugin including a NixOS test

This adds a `InfluxDBNotification` plugin which is configured as:

```
<influxdb>
  url = http://127.0.0.1:8086
  db = hydra
</influxdb>
```

which will write a notification for every finished job to the
configured database in InfluxDB looking like:

```
hydra_build_status,cached=false,job=job,jobset=default,project=sample,repo=default,result=success,status=success,system=x86_64-linux build_id="1",build_status=0i,closure_size=584i,duration=0i,main_build_id="1",queued=0i,size=168i 1564156212
```

src/lib/Hydra/Plugin/InfluxDBNotification.pm

@@ -0,0 +1,137 @@
+package Hydra::Plugin::InfluxDBNotification;
+
+use strict;
+use parent 'Hydra::Plugin';
+use HTTP::Request;
+# use JSON;
+use LWP::UserAgent;
+# use Hydra::Helper::CatalystUtils;
+
+sub toBuildStatusDetailed {
+    my ($buildStatus) = @_;
+    if ($buildStatus == 0) {
+        return "success";
+    }
+    elsif ($buildStatus == 1) {
+        return "failure";
+    }
+    elsif ($buildStatus == 2) {
+        return "dependency-failed";
+    }
+    elsif ($buildStatus == 4) {
+        return "cancelled";
+    }
+    elsif ($buildStatus == 6) {
+        return "failed-with-output";
+    }
+    elsif ($buildStatus == 7) {
+        return "timed-out";
+    }
+    elsif ($buildStatus == 9) {
+        return "unsupported-system";
+    }
+    elsif ($buildStatus == 10) {
+        return "log-limit-exceeded";
+    }
+    elsif ($buildStatus == 11) {
+        return "output-limit-exceeded";
+    }
+    elsif ($buildStatus == 12) {
+        return "non-deterministic-build";
+    }
+    else {
+        return "aborted";
+    }
+}
+
+sub toBuildStatusClass {
+    my ($buildStatus) = @_;
+    if ($buildStatus == 0) {
+        return "success";
+    }
+    elsif ($buildStatus == 3
+        || $buildStatus == 4
+        || $buildStatus == 8
+        || $buildStatus == 10
+        || $buildStatus == 11)
+    {
+        return "canceled";
+    }
+    else {
+        return "failed";
+    }
+}
+
+# Syntax
+# build_status,job=my-job status=failed,result=dependency-failed duration=123i
+#   |    -------------------- --------------  |
+#   |             |             |             |
+#   |             |             |             |
+#   +-----------+--------+-+---------+-+---------+
+#   |measurement|,tag_set| |field_set| |timestamp|
+#   +-----------+--------+-+---------+-+---------+
+sub createLine {
+    my ($measurement, $tagSet, $fieldSet, $timestamp) = @_;
+    my @tags = ();
+    foreach my $tag (sort keys %$tagSet) {
+        push @tags, "$tag=$tagSet->{$tag}";
+    }
+    my @fields = ();
+    foreach my $field (sort keys %$fieldSet) {
+        push @fields, "$field=$fieldSet->{$field}";
+    }
+    my $tags   = join(",", @tags);
+    my $fields = join(",", @fields);
+    return "$measurement,$tags $fields $timestamp";
+}
+
+sub buildFinished {
+    my ($self, $build, $dependents) = @_;
+    my $influxdb = $self->{config}->{influxdb};
+
+    # skip if we didn't configure
+    return unless defined $influxdb;
+    # skip if we didn't set the URL and the DB
+    return unless ref $influxdb eq 'HASH' and exists $influxdb->{url} and exists $influxdb->{db};
+
+    my @lines = ();
+    foreach my $b ($build, @{$dependents}) {
+        my $tagSet = {
+            status  => toBuildStatusClass($b->buildstatus),
+            result  => toBuildStatusDetailed($b->buildstatus),
+            project => $b->project->name,
+            jobset  => $b->jobset->name,
+            repo    => ($b->jobset->name =~ /^(.*)\.pr-/) ? $1 : $b->jobset->name,
+            job     => $b->job->name,
+            system  => $b->system,
+            cached  => $b->iscachedbuild ? "true" : "false",
+        };
+        my $fieldSet = {
+            # this line is needed to be able to query the statuses
+            build_status  => $b->buildstatus . "i",
+            build_id      => '"' . $b->id . '"',
+            main_build_id => '"' . $build->id . '"',
+            duration      => ($b->stoptime - $b->starttime) . "i",
+            queued        => ($b->starttime - $b->timestamp > 0 ? $b->starttime - $b->timestamp : 0) . "i",
+            closure_size  => ($b->closuresize // 0) . "i",
+            size          => ($b->size // 0) . "i",
+        };
+        my $line =
+          createLine("hydra_build_status", $tagSet, $fieldSet, $b->stoptime);
+        push @lines, $line;
+    }
+
+    my $payload = join("\n", @lines);
+    print STDERR "sending InfluxDB measurements to server $influxdb->{url}:\n$payload\n";
+
+    my $ua  = LWP::UserAgent->new();
+    my $req = HTTP::Request->new('POST',
+        "$influxdb->{url}/write?db=$influxdb->{db}&precision=s");
+    $req->header('Content-Type' => 'application/x-www-form-urlencoded');
+    $req->content($payload);
+    my $res = $ua->request($req);
+    print STDERR $res->status_line, ": ", $res->decoded_content, "\n"
+      unless $res->is_success;
+}
+
+1;