observability: log resolved span-start mode at processor init

The end-only skip is governed by AGENTEX_TRACING_SKIP_SPAN_START (default ON)
but was silent — an operator could only infer it from the absence of
start-export metrics. Emit a one-time INFO at processor init stating whether
span-start upsert is enabled or skipped, so the deployment's tracing mode is
visible in logs. Off the hot path (once per construction).

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

Author: NiteshDhanpal

src/agentex/lib/core/tracing/processors/sgp_tracing_processor.py

@@ -98,6 +98,11 @@ def __init__(self, config: SGPTracingProcessorConfig):
             disabled=disabled,
         )
         self.env_vars = EnvironmentVariables.refresh()
+        logger.info(
+            "SGP tracing span-start upsert %s (%s)",
+            "disabled — end-only ingest" if _skip_span_start_enabled() else "enabled",
+            _SKIP_SPAN_START_ENV,
+        )
 
     @override
     def on_span_start(self, span: Span) -> None:
@@ -134,6 +139,11 @@ def __init__(self, config: SGPTracingProcessorConfig):
             asyncio.AbstractEventLoop, AsyncSGPClient
         ] = weakref.WeakKeyDictionary()
         self.env_vars = EnvironmentVariables.refresh()
+        logger.info(
+            "SGP tracing span-start upsert %s (%s)",
+            "disabled — end-only ingest" if _skip_span_start_enabled() else "enabled",
+            _SKIP_SPAN_START_ENV,
+        )
 
     def _build_client(self) -> AsyncSGPClient:
         import httpx