docs(telemetry): fix head sampling at 1.0, remove configurable ratio

Document that head sampling is intentionally fixed at 100% and no longer
exposes a sampling_ratio config knob. A per-node ratio let nodes make
divergent keep/drop decisions for the same distributed trace, producing
broken/partial traces; pinning at 1.0 with a ParentBased sampler keeps
decisions coherent across the network. Volume reduction is delegated to
collector-side tail sampling.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

OpenTelemetryPlan/04-code-samples.md

@@ -53,8 +53,10 @@ public:
         bool useTls = false;
         std::string tlsCertPath;
 
-        // Sampling configuration
-        double samplingRatio = 1.0;  // 1.0 = 100% sampling
+        // Head sampling: fixed at 1.0 (sample everything), not config-driven.
+        // Keeps trace keep/drop decisions coherent across nodes; volume
+        // reduction is delegated to the collector's tail sampling.
+        double samplingRatio = 1.0;
 
         // Batch processor settings
         std::uint32_t batchSize = 512;