Log OTel init failures instead of silently swallowing them

Add LogExporterError() to exporter_interface.h (same pattern as
LogNegativeValue) so otel_exporter.cc can log via elog(WARNING)
without including postgres.h directly.

Author: iskakaushik

src/export/exporter_interface.h

@@ -81,6 +81,7 @@ class StatsExporter {
 
 // Allows PG logging of exceptional cases without postgres.h
 void LogNegativeValue(const std::string& column_name, int64_t value);
+void LogExporterError(const char* context, const char* message);
 
 // Expected usage:
 // void ProcessBatch(StatsExporter *exporter) {

src/export/otel_exporter.cc

@@ -440,7 +440,8 @@ bool OTelExporter::EstablishNewConnection() {
     logger = log_provider->GetLogger("pg_stat_ch", "pg_stat_ch_logs");
 
     return true;
-  } catch (const std::exception&) {
+  } catch (const std::exception& e) {
+    LogExporterError("OTel init failed", e.what());
     return false;
   }
 }

src/export/stats_exporter.cc

@@ -207,6 +207,10 @@ void ExportEventStats(const std::vector<PschEvent>& events, StatsExporter* expor
 
 }  // namespace
 
+void LogExporterError(const char* context, const char* message) {
+  elog(WARNING, "pg_stat_ch: %s: %s", context, message);
+}
+
 // Used to report negative values, which are not supported by OTel.
 void LogNegativeValue(const std::string& column_name, int64_t value) {
   static std::chrono::steady_clock::time_point last_log = {};