metrics: unpub Metrics::new()

As the user should not be able to create metrics instance otherwise
than by `get_metrics()` function, the `Metrics::new()` method shall
be set to pub(crate) visibility to support only internal usage.

scylla/src/observability/metrics.rs

@@ -209,7 +209,7 @@ pub struct Metrics {
 }
 
 impl Metrics {
-    pub fn new() -> Self {
+    pub(crate) fn new() -> Self {
         Self {
             errors_num: AtomicU64::new(0),
             queries_num: AtomicU64::new(0),