[DOC] List missing search_backpressure metric group

[lukas-vlcek]

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The Nodes Stats documentation is missing search_backpressure metric group in the table lists all available metric groups.

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.

# Do a clean checkout of OpenSearch from git and run a single node:
./gradlew clean run

# After it is ready open a web browser of your choice and point it to nodes stats API
# using `search_backpressure` as a metric group filter.

open -a "Google Chrome"  "http://localhost:9200/_nodes/stats/search_backpressure?pretty"

PR link: #7238