feat: capture and log attacker input from Telnet sessions

[vinayaktyagi10]

Summary

Adds real-time attacker input capture to the Telnet tarpit, logging
what attackers send during sessions as Prometheus metrics.

Changes

servers/telnet_pit.c

• Add non-blocking read() after each successful IAC write
• Sanitize captured input: replace non-printable bytes and IAC
  sequences (>126 or <32) with '.'
• Emit Telnet action <ip> <data> metric via sendMetric on each read
• Handle r==0 (client disconnected mid-session) with proper disconnect
  metric and cleanup

prometheus/main.go

• Add telnet_pit_input CounterVec with labels ip and data
• Handle action command in handleMetric switch statement
• Guard against malformed/short metric lines with len(fields) check

Why

The Telnet pit previously discarded all data sent by attackers.
This change captures that data in real-time, enabling researchers
to observe attacker behavior and build datasets of credentials and
commands used in the wild.

Testing

Tested locally by connecting via nc and typing input. Confirmed
Telnet action <ip> <data> lines appear in server output per
keystroke. Graceful shutdown confirmed still working after changes.

[vinayaktyagi10]

Update: Removed data from metric labels to avoid unbounded
Prometheus cardinality. Arbitrary attacker input as a label creates
a new time series per unique string, which causes memory pressure
under real attack load. Raw input is still logged via printf for
visibility.