mcap info: add channel count (#1243)

Merging #1051 with a small tweak to use Statistics's channel count if
available.

---------

Co-authored-by: peizhe.chen <peizhe.chen@horizon.cc>
Co-authored-by: pezy <urbancpz@gmail.com>

go/cli/mcap/cmd/info.go

@@ -174,9 +174,11 @@ func printInfo(w io.Writer, info *mcap.Info) error {
 		return err
 	}
 	if info.Statistics != nil {
+		fmt.Fprintf(buf, "channels: %d\n", info.Statistics.ChannelCount)
 		fmt.Fprintf(buf, "attachments: %d\n", info.Statistics.AttachmentCount)
 		fmt.Fprintf(buf, "metadata: %d\n", info.Statistics.MetadataCount)
 	} else {
+		fmt.Fprintf(buf, "channels: unknown\n")
 		fmt.Fprintf(buf, "attachments: unknown\n")
 		fmt.Fprintf(buf, "metadata: unknown\n")
 	}