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/sql/hydra.sql

@@ -324,6 +324,28 @@ create table BuildProducts (
 );
 
 
+create table BuildMetrics (
+    build         integer not null,
+    name          text not null,
+
+    unit          text,
+    value         double precision not null,
+
+    -- Denormalisation for performance: copy some columns from the
+    -- corresponding build.
+    project       text not null,
+    jobset        text not null,
+    job           text not null,
+    timestamp     integer not null,
+
+    primary key   (build, name),
+    foreign key   (build) references Builds(id) on delete cascade,
+    foreign key   (project) references Projects(name) on update cascade,
+    foreign key   (project, jobset) references Jobsets(project, name) on update cascade,
+    foreign key   (project, jobset, job) references Jobs(project, jobset, name) on update cascade
+);
+
+
 -- Cache for inputs of type "path" (used for testing Hydra), storing
 -- the SHA-256 hash and store path for each source path.  Also stores
 -- the timestamp when we first saw the path have these contents, which
@@ -590,6 +612,7 @@ create trigger NrBuildsFinished after insert or update or delete on Builds
 
 create index IndexBuildInputsOnBuild on BuildInputs(build);
 create index IndexBuildInputsOnDependency on BuildInputs(dependency);
+create index IndexBuildMetricsOnJobTimestamp on BuildMetrics(project, jobset, job, timestamp desc);
 create index IndexBuildProducstOnBuildAndType on BuildProducts(build, type);
 create index IndexBuildProductsOnBuild on BuildProducts(build);
 create index IndexBuildStepsOnBusy on BuildSteps(busy) where busy = 1;