Add metrics to documentation book
# Driver metrics
+
+During operation the driver collects various metrics.
+
+They can be accessed at any moment using `Session::get_metrics()`
+
+### Collected metrics:
+* Query latencies
+* Total number of nonpaged queries
+* Number of errors during nonpaged queries
+* Total number of paged queries
+* Number of errors during paged queries
+* Number of retries
+
+### Example
+```rust
+# extern crate scylla;
+# use scylla::Session;
+# use std::error::Error;
+# async fn check_only_compiles(session: &Session) -> Result<(), Box<dyn Error>> {
+let metrics = session.get_metrics();
+
+println!("Queries requested: {}", metrics.get_queries_num());
+println!("Iter queries requested: {}", metrics.get_queries_iter_num());
+println!("Errors occured: {}", metrics.get_errors_num());
+println!("Iter errors occured: {}", metrics.get_errors_iter_num());
+println!("Average latency: {}", metrics.get_latency_avg_ms().unwrap());
+println!(
+ "99.9 latency percentile: {}",
+ metrics.get_latency_percentile_ms(99.9).unwrap()
+);
+# Ok(())
+# }
+```
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