Add SLI for Rails to OpenBao calls

Why this MR ?

• Every Secrets Manager secret read, write and permission check crosses one HTTP hop from Rails to OpenBao
• We already have three Prometheus metrics on that hop, but they are plain counters and a histogram, so nothing can alert on them and they never reach our stage group error budget
• The documented way to get both is an application SLI - https://docs.gitlab.com/development/application_slis/
• Covers part of #611251

What does this MR do ?

• Adds the openbao_client_calls application SLI and records it from the same place the existing metrics are recorded
• Error rate only, no apdex - this hop's latency is dominated by OpenBao's four synchronous audit POSTs back into Rails, so any target worth setting would be red from day one
• The existing duration histogram still carries the percentiles
• Documents the two new counters

Notes for review

• The SLI module sits in lib/ rather than ee/lib/ because Gitlab::Instrumentation::Openbao is already a FOSS file and is the call site, and a FOSS file cannot reference an EE constant

References

• Issue - #611251
• MR that added the raw metrics - !251497 (merged)
• MR that fixed their outcomes - !252512 (merged)
• Runbooks MR that consumes this SLI - gitlab-com/runbooks!11333 (merged)
• Application SLI docs - https://docs.gitlab.com/development/application_slis/