Optionally output database table sizes after migrations complete (#38620)

Resolves #35314.

# Checklist for submitter

If some of the following don't apply, delete the relevant line.

- [x] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.
See [Changes
files](https://github.com/fleetdm/fleet/blob/main/docs/Contributing/guides/committing-changes.md#changes-files)
for more information.

- [x] Input data is properly validated, `SELECT *` is avoided, SQL
injection is prevented (using placeholders for values in statements)

## Testing

- [x] QA'd all new/changed functionality manually

cmd/fleet/prepare.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"bufio"
+	"encoding/json"
 	"fmt"
 	"os"
 
@@ -29,6 +30,8 @@ To setup Fleet infrastructure, use one of the available commands.
 	noPrompt := false
 	// Whether to enable developer options
 	dev := false
+	// Whether to show table stats before and after the migration
+	showTableStats := false
 
 	dbCmd := &cobra.Command{
 		Use:   "db",
@@ -68,6 +71,20 @@ To setup Fleet infrastructure, use one of the available commands.
 				}
 			}
 
+			if showTableStats {
+				defer func() {
+					stats, err := ds.GetTableRowCounts(cmd.Context())
+					if err != nil {
+						initFatal(err, "getting table stats")
+					}
+					statsAsJSON, err := json.Marshal(stats)
+					if err != nil {
+						initFatal(err, "encoding table row counts to JSON")
+					}
+					fmt.Printf("Table Row Counts: %s\n", statsAsJSON)
+				}()
+			}
+
 			switch status.StatusCode {
 			case fleet.NoMigrationsCompleted:
 				// OK
@@ -102,6 +119,7 @@ To setup Fleet infrastructure, use one of the available commands.
 
 	dbCmd.PersistentFlags().BoolVar(&noPrompt, "no-prompt", false, "disable prompting before migrations (for use in scripts)")
 	dbCmd.PersistentFlags().BoolVar(&dev, "dev", false, "Enable developer options")
+	dbCmd.PersistentFlags().BoolVar(&showTableStats, "with-table-stats", false, "Show approximate table row counts after migrations")
 
 	prepareCmd.AddCommand(dbCmd)
 	return prepareCmd