Fix FINEST-level error logging in GrpcExporter to include stack trace

The FINEST log in onError used string concatenation (+ e) which only
called toString(), missing the full stack trace. Changed to pass the
exception as the Throwable parameter so the logging framework can
properly render the complete stack trace at FINEST level.

Resolves #8098

Author: VamshikrishnaMonagari

exporters/common/src/main/java/io/opentelemetry/exporter/internal/grpc/GrpcExporter.java

@@ -133,7 +133,7 @@ private void onError(
             + e.getMessage(),
         e);
     if (logger.isLoggable(Level.FINEST)) {
-      logger.log(Level.FINEST, "Failed to export " + type + "s. Details follow: " + e);
+      logger.log(Level.FINEST, "Failed to export " + type + "s. Details follow:", e);
     }
     result.failExceptionally(FailedExportException.grpcFailedExceptionally(e));
   }