Add docs for monitoring TBS

[lahsivjar]

Documents the metrics produced by Tail-based sampler of APM Server and adds a few tips on how to monitor storage requirements.

Closes: elastic/apm-server#14996

CC: @elastic/obs-ds-intake-services (cannot ask for review directly)

[colleenmcginnis]

Some suggestions below!

[colleenmcginnis]

✨

[simitt]

@lahsivjar @colleenmcginnis is this going to be backported to 8.18+?

[lahsivjar]

@lahsivjar @colleenmcginnis is this going to be backported to 8.18+?

@simitt I talked with @raultorrecilla about this and the decision was to keep the docs for 9.x for now. If required, I can create a PR for 8.18 too (it will require a bit more details for the vlog metric and a description on how to add lsm and vlog metric to get total storage size so will need to create a new PR).

[simitt]

Based on what we just discussed in the team sync, let's link this PR to the gh card about improving TBS docs (once the card is created) and see how to best include the information. IMO we will need to also provider better docs for 8.18 as this will be were large customers are for a long while.

[lahsivjar]

IMO we will need to also provider better docs for 8.18 as this will be were large customers are for a long while.

@simitt Sounds good to me. I have created a PR for the 8.x branches (backported to 8.18, 8.17, and 8.16) with the updated docs.