Add support for using Redis MONITOR inputs files as the commands replayed during memtier execution

[fcostaoliveira]

This PR introduces first-class support for driving memtier_benchmark workloads from Redis MONITOR output files.

It adds:

• --monitor-input=FILE to load commands captured via redis-cli MONITOR
• --monitor-pattern={S|R} to control runtime command selection (Sequential by default, or Random)
• A new __monitor_c*__ placeholder family for --command:
  • __monitor_c1__, __monitor_c2__, … to replay a specific command from the monitor file
  • __monitor_c@__ to select commands at runtime (sequential or random)

This enables replaying real traffic and mixing captured workloads with synthetic commands using --command-ratio, while remaining compatible with both standalone and Redis Cluster deployments.

User-facing behavior

Replay a specific captured command on every request:

memtier_benchmark --monitor-input=monitor.txt --command=__monitor_c1__

Runtime-selected replay (sequential by default, random with --monitor-pattern=R):

memtier_benchmark --monitor-input=monitor.txt --command=__monitor_c@__ --monitor-pattern=R

Documentation

• Adds a new Using monitor input files section to the README
• Includes concrete usage examples
• Documents how to capture MONITOR output using redis-cli
• Links to the official Redis CLI MONITOR documentation