[8.x] terraform: support TBS in standalone_apm_server (backport #15337)

[mergify]

Motivation/summary

Support TBS in testing/infra/terraform/modules/standalone_apm_server

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

Run testing/benchmark with tfvars

apm_server_tail_sampling = true
apm_server_tail_sampling_storage_limit = "30GB"
run_standalone = true
standalone_apm_server_instance_size = "c6id.2xlarge"
worker_instance_type = "c6i.4xlarge"
standalone_apm_server_volume_type = "gp3"
standalone_apm_server_volume_size = 40

Output should contain a non-zero tbs_lsm_size e.g.

BenchmarkAgentAll-512        871         163274501 ns/op                 0 error_responses/sec        2205 errors/sec        19059 events/sec          995.0 gc_cycles        1049 max_goroutines        937528464 max_heap_alloc          4870724 max_heap_objects     1078747136 max_rss              22.21 mean_available_indexers            16854 metrics/sec           0 spans/sec      30967789 tbs_lsm_size  148710064 tbs_vlog_size                 0 txs/sec      436651105 B/op   4260866 allocs/op

Related issues

Follow up to #14985

Meta issue #14931

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This is an automatic backport of pull request #15337 done by [Mergify](https://mergify.com).