Do not start telemetry_poller when no metric exports are configured (#…

…2190)

While I was testing the shut down of `Electric.Connection.Supervisor`, I
saw errors getting logged when one of telemetry_poller's probes timed
out on a `GenServer.call()` to the connection manager. The thing is, we
didn't even need telemetry poller running in that scenario.

packages/sync-service/lib/electric/config.ex

@@ -109,6 +109,18 @@ defmodule Electric.Config do
     Application.fetch_env!(:electric, key)
   end
 
+  @doc """
+  True when at least one metric exporter is enabled.
+
+  This function is used to skip starting the Electric.Telemetry supervisor when there's no need
+  to capture periodic measurements. Useful in the dev and test environments.
+  """
+  def telemetry_export_enabled? do
+    not is_nil(Electric.Config.get_env(:telemetry_statsd_host)) or
+      not is_nil(Electric.Config.get_env(:prometheus_port)) or
+      Electric.Config.get_env(:call_home_telemetry?)
+  end
+
   @doc ~S"""
   Parse a PostgreSQL URI into a keyword list.
 

packages/sync-service/lib/electric/telemetry.ex

@@ -6,13 +6,17 @@ defmodule Electric.Telemetry do
   require Logger
 
   def start_link(init_arg) do
-    Supervisor.start_link(__MODULE__, init_arg, name: __MODULE__)
+    if Electric.Config.telemetry_export_enabled?() do
+      Supervisor.start_link(__MODULE__, init_arg, name: __MODULE__)
+    else
+      # Avoid starting the telemetry supervisor and its telemetry_poller child if we're not
+      # intending to export periodic measurements metrics anywhere.
+      :ignore
+    end
   end
 
   def init(opts) do
-    system_metrics_poll_interval =
-      Electric.Config.get_env(:system_metrics_poll_interval)
-
+    system_metrics_poll_interval = Electric.Config.get_env(:system_metrics_poll_interval)
     statsd_host = Electric.Config.get_env(:telemetry_statsd_host)
     prometheus? = not is_nil(Electric.Config.get_env(:prometheus_port))
     call_home_telemetry? = Electric.Config.get_env(:call_home_telemetry?)