chore: set timestamp field for otel appender (#150)

Signed-off-by: tison <wander4096@gmail.com>

Author: tisonkun

src/append/opentelemetry.rs

@@ -220,7 +220,10 @@ pub struct OpentelemetryLog {
 
 impl Append for OpentelemetryLog {
     fn append(&self, record: &Record, diagnostics: &[Box<dyn Diagnostic>]) -> anyhow::Result<()> {
+        let now = SystemTime::now();
+
         let mut log_record = self.logger.create_log_record();
+        log_record.set_timestamp(now);
         log_record.set_observed_timestamp(SystemTime::now());
         log_record.set_severity_number(log_level_to_otel_severity(record.level()));
         log_record.set_severity_text(record.level().as_str());