add metrics and implement the .stats command

This adds basic statement and connection metrics like SQLite (and
libSQL) have.

This is particularly useful to show that materialized views are working:

turso> create table t(a);
turso> insert into t(a) values (1) , (2), (3), (4), (5), (6), (7), (8), (9), (10);
turso> create materialized view v as select count(*) from t;
turso> .stats on
Stats display enabled.
turso> select count(*) from t;
┌───────────┐
│ count (*) │
├───────────┤
│        10 │
└───────────┘

Statement Metrics:
  Row Operations:
    Rows read:        10
    Rows written:     0
    [ ... other metrics ... ]

turso> select * from v;
┌───────────┐
│ count (*) │
├───────────┤
│        10 │
└───────────┘

Statement Metrics:
  Row Operations:
    Rows read:        1
    Rows written:     0
    [ ... other metrics ... ]

cli/commands/mod.rs

@@ -3,8 +3,8 @@ pub mod import;
 
 use args::{
     CwdArgs, DbConfigArgs, EchoArgs, ExitArgs, HeadersArgs, IndexesArgs, LoadExtensionArgs,
-    NullValueArgs, OpcodesArgs, OpenArgs, OutputModeArgs, SchemaArgs, SetOutputArgs, TablesArgs,
-    TimerArgs,
+    NullValueArgs, OpcodesArgs, OpenArgs, OutputModeArgs, SchemaArgs, SetOutputArgs, StatsArgs,
+    TablesArgs, TimerArgs,
 };
 use clap::Parser;
 use import::ImportArgs;
@@ -78,6 +78,9 @@ pub enum Command {
     /// Print or set the current configuration for the database. Currently ignored.
     #[command(name = "dbconfig", display_name = ".dbconfig")]
     DbConfig(DbConfigArgs),
+    /// Display database statistics
+    #[command(name = "stats", display_name = ".stats")]
+    Stats(StatsArgs),
     /// List vfs modules available
     #[command(name = "vfslist", display_name = ".vfslist")]
     ListVfs,