[Logstash - Investigation] Concurrent `PipelineAction::Create` at boot can raise `MetricStore` `KeyError: key not found`

When several pipelines start concurrently, one pipeline's `Create` intermittently fails with `KeyError: key not found` while registering flow metrics. Evidence points to a race condition in metric registration, needs investigation to confirm and to implement the proper fix.

## Evidence
- Backtrace (key frames):
```
KeyError: key not found
concurrent/map.rb:in `raise_fetch_no_key' / `fetch' / `fetch_or_store'
logstash/instrument/metric_store.rb:in `block in fetch_or_store'
logstash/instrument/collector.rb:in `register?'
org/logstash/execution/AbstractPipelineExt.java:in `initialize_flow_metrics'
logstash/agent.rb:in `block in converge_state'
```
- **Intermittent** (a different pipeline each time). Signature of a timing race, not a config/compile error.
- At boot each `Create` runs on its pwm, so many threads hit the single process-wide collector at once.

## Code context

The random, intermittent pipeline error looks like a race condition, but the raised exception comes from a call protected by `@structured_lookup_mutex`, so I think every structured-store write is serialized. A call serialized that way would behave single-threaded and normally never raise. Resolving that contradiction is the goal of this issue.

- The `fetch_or_store` call in Logstash that raises — [metric_store.rb#L85](https://github.com/elastic/logstash/blob/main/logstash-core/lib/logstash/instrument/metric_store.rb#L85)
- Where it throws in `concurrent-ruby` (`~> 1.3`) — [`raise_fetch_no_key` map.rb#L327](https://github.com/ruby-concurrency/concurrent-ruby/blob/v1.3.5/lib/concurrent-ruby/concurrent/map.rb#L327)

Start at logstash-core/lib/logstash/instrument/metric_store.rb#L85 and trace the synchronized fetch_or_store call through collector.rb#register? and concurrent-ruby's map.rb#L327. Reproduce or inspect concurrent pipeline boot through AbstractPipelineExt#initialize_flow_metrics and agent.rb#converge_state, focusing on the reported mutex contradiction. Done means the race cause is confirmed and the appropriate fix prevents the intermittent KeyError.

Written by the indexing model from the issue text.