docs: Remove metrics signal

Author: Techassi

modules/concepts/pages/observability/telemetry.adoc

@@ -3,7 +3,6 @@
 Since SDP 25.7.0, all Stackable operators emit telemetry data in the following ways:
 
 * logs (via console, file, and OTLP export)
-* metrics (not yet available, will most likely be an OTLP export)
 * traces (via OTLP export)
 
 Every signal can be toggled and customized using Helm values.