Provide fallback for Observation KeyValues

[jonatan-ivanov]

Similarly to #4994 this PR adds a fallback value to all the KeyValues used by DefaultMongoHandlerObservationConvention.

TODO:

• Read the Spring Data contribution guidelines
• Use the code formatters provided here
• Add test cases
• Added myself as author in the headers of the classes I touched
• Maybe rebasing it on top of 4.4.x

[shakuzen]

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

[mp911de]

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

[mp911de]

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.