chore(llmobs): fix flush telemetry metric name (#13292)

Fixes telemetry metric name `user_flushes` to `user_flush` which is what
we're expecting/allowing on our metric ingest.

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Author: Yun-Kim

ddtrace/llmobs/_telemetry.py

@@ -28,7 +28,7 @@ class LLMObsTelemetryMetrics:
     ANNOTATIONS = "annotations"
     EVALS_SUBMITTED = "evals_submitted"
     SPANS_EXPORTED = "spans_exported"
-    USER_FLUSHES = "user_flushes"
+    USER_FLUSHES = "user_flush"
     INJECT_HEADERS = "inject_distributed_headers"
     ACTIVATE_HEADERS = "activate_distributed_headers"