Describe the bug
The DefaultMeterObservationHandler performs unnecessary tag sorting for every request, leading to potential performance overhead. This occurs during the creation of Tags objects in both the onStart and onStop methods, even when the underlying key-value pairs are the same. This results in repeated sorting operations, which are relatively expensive.

The issue lies in the way Tags objects are created. Even though the same set of low-cardinality key-value pairs is used repeatedly, a new List of KeyValue objects is created for each invocation in both onStart and onStop. This new List is then used to create a new Tags object, which triggers the sorting process.

While sorting is necessary for the internal representation of Tags, it shouldn't be performed on every request when the key-value pairs haven't changed.

Proposed Solutions:

Cache Tags Objects: Implement a caching mechanism to store and reuse Tags objects based on the set of key-value pairs.

private final ConcurrentMap<List<KeyValue>, Tags> tagsCache = new ConcurrentHashMap<>(); 



private Tags getCachedTags(Observation.Context context) {

List<KeyValue> keyValues = context.getLowCardinalityKeyValues();

return tagsCache.computeIfAbsent(keyValues, this::createTags);

} 

Environment

• Micrometer version [e.g. 1.14.0]
• Micrometer registry [e.g. prometheus]
• OS: [e.g. macOS]
• Java version: [e.g. output of java -version]

To Reproduce
How to reproduce the bug:

1. Set up a Spring Boot application (or any application using Micrometer) that utilizes the DefaultMeterObservationHandler for collecting metrics.
2. Create an endpoint or service that generates consistent low-cardinality key-value pairs in the Observation.Context.
3. Instrument this endpoint or service with Micrometer's Observation API.
4. Make repeated calls to the endpoint or service, ensuring the same set of key-value pairs is generated with each call.

Expected behavior
A clear and concise description of what you expected to happen.

The DefaultMeterObservationHandler should cache and reuse Tags objects for identical sets of low-cardinality key-value pairs.
Tag sorting should only occur once for each unique set of key-value pairs.
Subsequent requests with the same key-value pairs should retrieve the cached Tags object, avoiding redundant sorting.
Performance should improve, especially under high load with repeated calls generating the same tags.

Additional context
Add any other context about the problem here, e.g. related issues.