9.10.0: Always initialise GovukPrometheusExporter.

Some client apps have started to assume that the Prometheus "exporter"
process (i.e. the process that aggregates counters in multi-process
apps) is always available, including in tests.

Now that we handle the case where the `/metrics` port is already taken,
there's really no reason not to initialise GovukPrometheusExporter.

Remove the confusing heuristics in
`GovukPrometheusExporter.should_configure` and just always attempt to
initialise unless we're running under `rails console`. When the init
fails, we just print a warning and continue anyway, so this should be a
net win for robustness.

If we find edge cases where this still needs to be configurable then I
propose we address those if/when we find and understand them. I expect
we won't need to, though.

CHANGELOG.md

@@ -1,3 +1,7 @@
+# 9.10.0
+
+* Simplify the logic for deciding whether to initialize `GovukPrometheusExporter`. `GovukPrometheusExporter` provides the `/metrics` webserver and "exporter" process for aggregating counters in multi-process apps. govuk_app_config will now always attempt to initialize GovukPrometheusExporter except when running under `rails console`. The `GOVUK_PROMETHEUS_EXPORTER` environment variable no longer has any effect.
+
 # 9.9.2
 
 * Add single cookie consent api URLs ([#355](https://github.com/alphagov/govuk_app_config/pull/355))

lib/govuk_app_config/govuk_prometheus_exporter.rb

@@ -42,15 +42,7 @@ def default_labels(_env, _result)
   end
 
   def self.should_configure
-    # Allow us to force the Prometheus Exporter for persistent Rake tasks...
-    if ENV["GOVUK_PROMETHEUS_EXPORTER"] == "force"
-      true
-    elsif File.basename($PROGRAM_NAME) == "rake" ||
-        defined?(Rails) && (Rails.const_defined?("Console") || Rails.env == "test")
-      false
-    else
-      ENV["GOVUK_PROMETHEUS_EXPORTER"] == "true"
-    end
+    !(defined?(Rails) && Rails.const_defined?("Console"))
   end
 
   def self.configure(collectors: [], default_aggregation: PrometheusExporter::Metric::Histogram)
@@ -81,9 +73,7 @@ def self.configure(collectors: [], default_aggregation: PrometheusExporter::Metr
 
     begin
       server = PrometheusExporter::Server::WebServer.new bind: "0.0.0.0", port: 9394
-
-      collectors.each { |collector| server.collector.register_collector(collector.new) }
-
+      collectors.each { |c| server.collector.register_collector(c.new) }
       server.start
 
       if defined?(Rails)
@@ -94,7 +84,7 @@ def self.configure(collectors: [], default_aggregation: PrometheusExporter::Metr
         Sinatra.use SinatraMiddleware
       end
     rescue Errno::EADDRINUSE
-      warn "Could not start Prometheus metrics server as address already in use."
+      warn "not starting Prometheus metrics server: address already in use"
     end
   end
 end