The synocli-monitor package provides some monitoring related tools to advanced Linux users:

• busybox
  This package includes some process utilities provided by busybox: iostat, pgrep, pmap, watch (and pstree on DSM 5). [GPL]

• ionice
  ionice - set or get process I/O scheduling class and priority. ionice it part of util-linux, a random collection of Linux utilities. [GPLv2]

• lsof
  lsof is a command listing open files. [License]

• nmon
  nmon (Nigel's performance Monitor) is a performance monitoring tool for Linux.
  njmon is similar but saves data to JSON format for a new generation of online time-series databases and web-browser graphing. [GPLv3]

• iperf2
  A tool for measuring TCP and UDP network performance, based on iperf 2.0.5. [BSD]

• iperf3
  The iperf series of tools perform active measurements to determine the maximum achievable bandwidth on IP networks. It supports tuning of various parameters related to timing, protocols, and buffers. For each test it reports the measured throughput, loss, and other parameters. [three clause BSD]

• htop (you need to call this with full path, as older version of htop is included in DSM 5.1 and newer)
  Interactive text-mode process viewer for Unix systems. The package is intended for DSM-5.0 or earlier where it's not available. [GPLv2]

• cpulimit
  CPU usage limiter for Linux. [GPLv2+]

• bandwhich
  CLI utility for displaying current network utilization by process, connection and remote IP/hostname. [MIT]

• btm (bottom)
  A customizable cross-platform graphical process/system monitor for the terminal. [MIT]

• procs
  A modern replacement for ps written in Rust. [MIT]

• net-snmp
  Simple Network Management Protocol (SNMP) is a widely used protocol for monitoring the health and welfare of network equipment (eg. routers), computer equipment and even devices like UPSs. Net-SNMP is a suite of applications used to implement SNMP v1, SNMP v2c and SNMP v3 using both IPv4 and IPv6.
  Only command line tools are included in synocli-monitor package. The daemon snmpd is provided by Synology and running on DSM when activated. Synology provides a single command line tool snmpwalk. That's why you have to use full qualified name if you want to run snmpwalk provided by this package. [MIT]

• lm-sensors
  lm-sensors provides user-space support for the hardware monitoring drivers in Linux 2.6.5 and later. [GPLv2]