Add changelog for SignalFx CPU count translation

Assisted-by: GPT-5

Author: dmitryax

.chloggen/signalfx-cpu-num-processors-logical-count.yaml

@@ -0,0 +1,8 @@
+change_type: breaking
+component: exporter/signalfx
+note: "Translate `cpu.num_processors` from `system.cpu.logical.count`."
+issues: [49296]
+subtext: |
+  The exporter no longer derives `cpu.num_processors` by counting per-core `system.cpu.time` series.
+  Enable `system.cpu.logical.count` in the hostmetrics receiver CPU scraper to emit `cpu.num_processors` with the default SignalFx translations.
+change_logs: [user]