Adding collectd and sysstat tool to comparsion docs (#8259)

Summary:
## What this PR does

This PR updates the comparison document to include two additional tools: **sysstat (sar)** and **collectd**, in response to [https://github.com/facebookincubator/below/issues/8255](https://github.com/facebookincubator/below/issues/8255).

Both tools are commonly used in system observability and performance monitoring, and adding them helps provide a more complete view of the ecosystem for potential users.

## Why this is needed

The request in issue [https://github.com/facebookincubator/below/issues/8255](https://github.com/facebookincubator/below/issues/8255) highlighted the absence of `sysstat` and `collectd` in the current comparison doc. Including them:

- Acknowledges widely adopted monitoring tools in the Linux ecosystem.
- Helps users understand how `below` compares with both daemon-based collectors and CLI-based utilities.

## Reference

Closes: #8255

Pull Request resolved: #8259

Reviewed By: boyuni

Differential Revision: D79807468

Pulled By: dschatzberg

fbshipit-source-id: 488024dc0d5cbfe9a3290d0fe8fe1c09d9d8c829

Author: nmn3m

docs/comparison.md

@@ -51,6 +51,40 @@ https://htop.dev/
 * No cgroup awareness
 * Limited host-level stats
 
+## sysstat (sar)
+
+https://github.com/sysstat/sysstat
+
+* CLI-based tools like `sar`, `iostat`, `pidstat`, and others
+* Collects a wide range of system performance metrics (CPU, memory, IO, etc.)
+* Mature and widely used for historical performance tracking
+* Data can be collected periodically via `cron` or systemd timers
+* Data stored in a binary format, accessible through CLI
+
+### Drawbacks
+
+* No interactive or web interface
+* Lacks container or cgroup-specific visibility
+* Requires parsing and scripting to extract structured data
+* Binary logs are not human-readable without `sar`
+
+## collectd
+
+https://collectd.org/
+
+* Daemon-based metrics collector
+* Plugin architecture for system and application metrics (disk, memory, network, etc.)
+* Can push metrics to various backends (Graphite, InfluxDB, Prometheus via exporter)
+* Lightweight and extensible with community plugins
+* Suitable for long-term storage and visualization setups
+
+### Drawbacks
+
+* No built-in UI or terminal interface
+* Configuration can be complex
+* Requires external tooling for data visualization
+* No direct cgroup or container integration by default
+
 ## below
 
 https://github.com/facebookincubator/below