Expose prometheus endpoint per metric in dashboard

Adds a new stage to dashboard processesing step for the
EnvironmentsController::metrics_dashboard endpoint.

Allows the front end to avoid generating the endpoint
unitutive string mutations.

3 files changed, 47 insertions, 1 deletions

diff --git a/lib/gitlab/metrics/dashboard/base_service.rb b/lib/gitlab/metrics/dashboard/base_service.rb
index 94aabd0466c..ce6ecf933a1 100644
--- a/lib/gitlab/metrics/dashboard/base_service.rb
+++ b/lib/gitlab/metrics/dashboard/base_service.rb
@@ -7,12 +7,13 @@ module Gitlab
     module Dashboard
       class BaseService < ::BaseService
         DASHBOARD_LAYOUT_ERROR = Gitlab::Metrics::Dashboard::Stages::BaseStage::DashboardLayoutError
+        MISSING_QUERY_ERROR = Gitlab::Metrics::Dashboard::Stages::EndpointInserter::MissingQueryError
 
         def get_dashboard
           return error("#{dashboard_path} could not be found.", :not_found) unless path_available?
 
           success(dashboard: process_dashboard)
-        rescue DASHBOARD_LAYOUT_ERROR => e
+        rescue DASHBOARD_LAYOUT_ERROR, MISSING_QUERY_ERROR => e
           error(e.message, :unprocessable_entity)
         end
 
diff --git a/lib/gitlab/metrics/dashboard/processor.rb b/lib/gitlab/metrics/dashboard/processor.rb
index dd986020693..a33a010ad97 100644
--- a/lib/gitlab/metrics/dashboard/processor.rb
+++ b/lib/gitlab/metrics/dashboard/processor.rb
@@ -11,11 +11,13 @@ module Gitlab
         SYSTEM_SEQUENCE = [
           Stages::CommonMetricsInserter,
           Stages::ProjectMetricsInserter,
+          Stages::EndpointInserter,
           Stages::Sorter
         ].freeze
 
         PROJECT_SEQUENCE = [
           Stages::CommonMetricsInserter,
+          Stages::EndpointInserter,
           Stages::Sorter
         ].freeze
 
diff --git a/lib/gitlab/metrics/dashboard/stages/endpoint_inserter.rb b/lib/gitlab/metrics/dashboard/stages/endpoint_inserter.rb
new file mode 100644
index 00000000000..797df14b0e7
--- /dev/null
+++ b/lib/gitlab/metrics/dashboard/stages/endpoint_inserter.rb
@@ -0,0 +1,43 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Metrics
+    module Dashboard
+      module Stages
+        class EndpointInserter < BaseStage
+          MissingQueryError = Class.new(StandardError)
+
+          def transform!
+            for_metrics do |metric|
+              metric[:prometheus_endpoint_path] = endpoint_for_metric(metric)
+            end
+          end
+
+          private
+
+          def endpoint_for_metric(metric)
+            Gitlab::Routing.url_helpers.prometheus_api_namespace_project_environment_path(
+              project.namespace,
+              project,
+              environment,
+              proxy_path: query_type(metric),
+              query: query_for_metric(metric)
+            )
+          end
+
+          def query_type(metric)
+            metric[:query] ? :query : :query_range
+          end
+
+          def query_for_metric(metric)
+            query = metric[query_type(metric)]
+
+            raise MissingQueryError.new('Missing required metric key: one of :query or :query_range') unless query
+
+            query
+          end
+        end
+      end
+    end
+  end
+end