Add prometheus metrics at /api/v1/metrics

src/invidious.cr

@@ -186,6 +186,7 @@ end
 
 if CONFIG.statistics_enabled
   Invidious::Jobs.register Invidious::Jobs::StatisticsRefreshJob.new(PG_DB, SOFTWARE)
+  add_handler Metrics::RouteMetricsCollector.new
 end
 
 if (CONFIG.use_pubsub_feeds.is_a?(Bool) && CONFIG.use_pubsub_feeds.as(Bool)) || (CONFIG.use_pubsub_feeds.is_a?(Int32) && CONFIG.use_pubsub_feeds.as(Int32) > 0)

src/invidious/helpers/helpers.cr

@@ -198,3 +198,11 @@ def get_playback_statistic
 
   return tracker.as(Hash(String, Int64 | Float64))
 end
+
+def to_prometheus_metrics(metrics_struct : Hash(String, Number)) : String
+  return String.build do |str|
+    metrics_struct.each.each do |key, value|
+      str << key << " " << value << "\n"
+    end
+  end
+end

src/invidious/jobs/statistics_refresh_job.cr

@@ -27,6 +27,11 @@ class Invidious::Jobs::StatisticsRefreshJob < Invidious::Jobs::BaseJob
     "playback" => {} of String => Int64 | Float64,
   }
 
+  STATISTICS_PROMETHEUS = {
+    "invidious_updated_at"                => Time.utc.to_unix,
+    "invidious_last_channel_refreshed_at" => 0_i64,
+  }
+
   private getter db : DB::Database
 
   def initialize(@db, @software_config : Hash(String, String))
@@ -59,6 +64,9 @@ class Invidious::Jobs::StatisticsRefreshJob < Invidious::Jobs::BaseJob
     users["activeHalfyear"] = Invidious::Database::Statistics.count_users_active_6m
     users["activeMonth"] = Invidious::Database::Statistics.count_users_active_1m
 
+    STATISTICS_PROMETHEUS["invidious_updated_at"] = Time.utc.to_unix
+    STATISTICS_PROMETHEUS["invidious_last_channel_refreshed_at"] = Invidious::Database::Statistics.channel_last_update.try &.to_unix || 0_i64
+
     STATISTICS["metadata"] = {
       "updatedAt"              => Time.utc.to_unix,
       "lastChannelRefreshedAt" => Invidious::Database::Statistics.channel_last_update.try &.to_unix || 0_i64,

src/invidious/metrics.cr

@@ -0,0 +1,46 @@
+# Module containing an optional Kemal handler, which can be used
+# to collect metrics about the usage of various Invidious routes.
+module Metrics
+  record MetricLabels, request_method : String, request_route : String, response_code : Int32
+
+  class RouteMetricsCollector < Kemal::Handler
+    # Counts how many times a given route was used
+    @@num_of_request_counters = Hash(MetricLabels, Int64).new
+    # Counts how much time was used to handle requests to each route
+    @@request_duration_seconds_sums = Hash(MetricLabels, Float32).new
+
+    def self.num_of_request_counters
+      return @@num_of_request_counters
+    end
+
+    def self.request_duration_seconds_sums
+      return @@request_duration_seconds_sums
+    end
+
+    def call(context : HTTP::Server::Context)
+      request_handling_started = Time.utc
+      begin
+        call_next(context)
+      ensure
+        request_handling_finished = Time.utc
+        request_path = context.route.path
+        request_method = context.request.method
+        seconds_spent_handling = (request_handling_finished - request_handling_started).to_f
+        response_status = context.response.status_code.to_i
+
+        LOGGER.trace("Collecting metrics: handling #{request_method} #{request_path} took #{seconds_spent_handling}s and finished with status #{response_status}")
+        metric_key = MetricLabels.new request_path, request_method, response_status
+
+        unless @@num_of_request_counters.has_key?(metric_key)
+          @@num_of_request_counters[metric_key] = 0
+        end
+        @@num_of_request_counters[metric_key] += 1
+
+        unless @@request_duration_seconds_sums.has_key?(metric_key)
+          @@request_duration_seconds_sums[metric_key] = 0.0
+        end
+        @@request_duration_seconds_sums[metric_key] += seconds_spent_handling
+      end
+    end
+  end
+end

src/invidious/routes/api/v1/misc.cr

@@ -1,3 +1,5 @@
+require "../../../metrics.cr"
+
 module Invidious::Routes::API::V1::Misc
   # Stats API endpoint for Invidious
   def self.stats(env)
@@ -26,6 +28,39 @@ module Invidious::Routes::API::V1::Misc
     end
   end
 
+  def self.metrics(env)
+    if !CONFIG.statistics_enabled
+      env.response.status_code = 204
+      return
+    end
+
+    env.response.content_type = "text/plain"
+
+    return String.build do |str|
+      Metrics::RouteMetricsCollector.num_of_request_counters.each do |metric_labels, value|
+        str << "http_requests_total{"
+        str << "method=\"" << metric_labels.request_method << "\" "
+        str << "route=\"" << metric_labels.request_route << "\" "
+        str << "response_code=\"" << metric_labels.response_code << "\""
+        str << "} "
+        str << value << "\n"
+      end
+
+      Metrics::RouteMetricsCollector.request_duration_seconds_sums.each do |metric_labels, value|
+        str << "http_request_duration_seconds_sum{"
+        str << "method=\"" << metric_labels.request_method << "\" "
+        str << "route=\"" << metric_labels.request_route << "\" "
+        str << "response_code=\"" << metric_labels.response_code << "\""
+        str << "} "
+        str << value << "\n"
+      end
+
+      Invidious::Jobs::StatisticsRefreshJob::STATISTICS_PROMETHEUS.each.each do |key, value|
+        str << key << " " << value << "\n"
+      end
+    end
+  end
+
   # APIv1 currently uses the same logic for both
   # user playlists and Invidious playlists. This means that we can't
   # reasonably split them yet. This should be addressed in APIv2

src/invidious/routing.cr

@@ -310,6 +310,7 @@ module Invidious::Routing
 
       # Misc
       get "/api/v1/stats", {{namespace}}::Misc, :stats
+      get "/api/v1/metrics", {{namespace}}::Misc, :metrics
       get "/api/v1/playlists/:plid", {{namespace}}::Misc, :get_playlist
       get "/api/v1/auth/playlists/:plid", {{namespace}}::Misc, :get_playlist
       get "/api/v1/mixes/:rdid", {{namespace}}::Misc, :mixes