Add API support for providing fallback for KeyValues

[jonatan-ivanov]

Applying KeyValues conditionally can lead into issues for example Prometheus require that all time-series (observation) with the same name have the same set of labels (low cardinality key names).

For example this can cause an error on if both branches (color = null and color = "red") are executed.

if (color != null) {
    observation.lowCardinalityKeyValue("color", color);
}

Instead of it, users should do something like this:

observation.lowCardinalityKeyValue("color", color != null ? color : "unknown");

We should make this easier for the users and harder to make this mistake.

See:

• Provide fallback for Observation KeyValues spring-projects/spring-data-mongodb#5020 (comment)
• https://gist.github.com/mp911de/6729ca327326d072820a1a0eb225c4a6

Validation could be also added: main...jonatan-ivanov:micrometer:validate-keyvalues