can we print the message with its lenght indicator ?

[Achuthen]

rather than just

F2 Primary Account Number............................: 6442****8074

how to print something like this
F2 Primary Account Number............................: [016] 6442****8074

the spec is as such
2: field.NewString(&field.Spec{
Length: 19,
Description: "Primary Account Number",
Enc: encoding.ASCII,
Pref: prefix.ASCII.LL,
}),