uses nanos precision for timestamp when submitting metrics to influxdb (#20623)

Current datapoint_info! is apparently overwriting itself when run inside
a loop. For example in
https://github.com/solana-labs/solana/blob/005d6863f/core/src/window_service.rs#L101-L107
only one of the slots will show up in influxdb.

This is apparently because of metrics code using milliseconds as the
timestamp, as mentioned here:
https://github.com/solana-labs/solana/issues/19789#issuecomment-922482013

metrics/src/datapoint.rs

@@ -1,9 +1,9 @@
-use std::fmt;
+use std::{fmt, time::SystemTime};
 
 #[derive(Clone, Debug)]
 pub struct DataPoint {
     pub name: &'static str,
-    pub timestamp: u64,
+    pub timestamp: SystemTime,
     pub fields: Vec<(&'static str, String)>,
 }
 
@@ -11,7 +11,7 @@ impl DataPoint {
     pub fn new(name: &'static str) -> Self {
         DataPoint {
             name,
-            timestamp: solana_sdk::timing::timestamp(),
+            timestamp: SystemTime::now(),
             fields: vec![],
         }
     }