TBS: Document monitoring of disk space used by Tail Based Sampling in public docs.

[lahsivjar]

Configuring storage is crucial for stable Tail-Based-Sampling performance. We already emit metrics for monitoring disk usage by TBS but we have no public documentation around this. Having a public documentation for this will allow users to create dashboards and be on top of their storage needs for TBS. Example:

• Total TBS database size: last_value('golang.apm-server.apm-server.sampling.tail.storage.lsm_size', kql='"beat.stats.apm_server.sampling.tail.storage.lsm_size": *') + last_value('beat.stats.apm_server.sampling.tail.storage.value_log_size', kql='"beat.stats.apm_server.sampling.tail.storage.value_log_size": *')
• LSM size: last_value('golang.apm-server.apm-server.sampling.tail.storage.lsm_size', kql='"beat.stats.apm_server.sampling.tail.storage.lsm_size": *')
• vlog size: last_value('beat.stats.apm_server.sampling.tail.storage.value_log_size', kql='"beat.stats.apm_server.sampling.tail.storage.value_log_size": *')