CQL Configuration set through storage.cql.internal overrides all values

[cdegroc]

We've observed that setting any value in storage.cql.internal.string-configuration will override storage.cql.metrics.node-enabled, even when node-enabled is not redefined in the string-configuration.

How to reproduce:

1. Set node enabled metrics in the programmatic-configuration. For instance, set CQLConfigOptions.METRICS_NODE_ENABLED to pool.open-connections,pool.available-streams,pool.in-flight
2. Set something in the string-configuration. For instance, set CQLConfigOptions.STRING_CONFIGURATION to datastax-java-driver { basic.request.timeout = 5000 milliseconds }

Metrics won't be enabled (and printing Cassandra driver's final composite configuration would include advanced.metrics.node.enabled=[]).

I find it misleading considering what the documentation says:

The configurations are composed in the next order (sorted by priority in descending order): file-configuration, resource-configuration, string-configuration, url-configuration, base-programmatic-configuration (which is controlled by base-programmatic-configuration-enabled property). Configurations with higher priority always overwrite configurations with lower priority. I.e. if the same configuration parameter is used in both file-configuration and string-configuration the configuration parameter from file-configuration will be used and configuration parameter from string-configuration will be ignored.

I think this happens because DriverConfigLoader's fromString/fromClasspath/fromFile} always load the default reference.conf file as a fallback. So when a value is set in CQLConfigOptions.STRING_CONFIGURATION, all default values defined in reference.conf will be set there as well.

I'm not sure whether we should update the documentation or consider this as a bug.

[cdegroc]

Same issue was reported in #3005 and fixed in #3008.