fix(obs): remove invalid processors block from tempo metrics_generator

Tempo 2.7.2 removed `processors` from the top-level metrics_generator
config; the field is only valid under `overrides.defaults.metrics_generator`.
The setting was already present there, so this only removes the now-rejected
duplicate at the top level.

Closes part of #601

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

infra/observability/tempo/tempo.yml

@@ -36,9 +36,6 @@ metrics_generator:
       source: tempo
   storage:
     path: /var/tempo/generator/wal
-  processors:
-    - service-graphs
-    - span-metrics
 
 # Tempo HTTP API (port 3200) is unauthenticated. Access is controlled entirely
 # by network isolation: only Grafana (on obs-net) should reach this port.