I wonder if "UNKNOWN" is a better default value for most of these rather than none.

I would like to have a bit more encapsulated API for a more declarative approach to reduce the likelihood for using the API wrong. Something along the lines of:

// TODO: it would be nice to have a getKeyName() method returning KeyName from KeyValue for usage in e.g. KeyName[] getLowCardinalityKeyNames() 
static KeyValue DB_SYSTEM = KeyValue.of("db.system", "mongodb");


static SmartKeyName<ConnectionString> DB_CONNECTION_STRING = SmartKeyName.required("db.connection_string",
		ConnectionString::getConnectionString);

later:

@Nullable ConnectionString connectionString = context.getConnectionString();

KeyValues keyValues = KeyValues.of(MongoObservation.DB_SYSTEM,
		MongoObservation.DB_CONNECTION_STRING.valueOf(connectionString));

You can find SmartKeyName (needs probably a better name) at
https://gist.github.com/mp911de/6729ca327326d072820a1a0eb225c4a6

@shakuzen

I wonder if "UNKNOWN" is a better default value for most of these rather than none.

I think so, I can also make the fallback mandatory so it needs to be decided for every key-value.

@mp911de

I would like to have a bit more encapsulated API for a more declarative approach [...]

We can consider improving on this in Micrometer 1.16 (let me talk to @shakuzen about it) but I think this should be considered as a bug and should be fixed in earlier versions where Micrometer 1.16 will not be available I think (unless you want to copy some code from there).

I see your approach and I like it more than concatenating KeyValues. With a refined approach regarding value extraction, we might get a chance for improvement.