Add guards for the `MetricStore`

The Metric store would benefit from some guards to protect the data.
- The current implementation of the `MetricStore` can grow unbounded, we should add guards on the size of it to make sure its stays under control and log that information.
- We can create namspace in unknown location we might want to secure or protect them, this is from a discussion from this PR https://github.com/elastic/logstash/pull/4602#discussion_r51735098

Start by locating the MetricStore implementation and reviewing how it stores metrics and creates namespaces. Read the linked pull request discussion for the namespace concern, then identify existing logging and test conventions. Done means bounded storage, protected namespace creation, and logging for guard-triggered conditions, with tests covering the new behavior.

Written by the indexing model from the issue text.