Improve tools for measuring realtime metrics

[farshidtz]

• Reword the tool descriptions to fix the grammar.
• Explain the examples before they are given, rather than after.

Originally posted by @farshidtz in https://github.com/canonical/real-time-ubuntu-docs/issues/30#issuecomment-2134774300

Link: https://canonical-real-time-ubuntu-documentation.readthedocs-hosted.com/en/latest/reference/real-time-metrics-tools/