Fixes #2158

This addresses issues which may arise when using a database implementation which doesn't construct $wpdb->queries in the same way as core does, namely when a logged query is not an indexed array or it it doesn't have a float at index 1. This can happen when using a custom save_query_callback in HyperDB, for example. This essentially builds upon the existing type checking for odd database implementations which don't define $wpdb->queries. In both cases, notices are now emitted before aborting.

If $wpdb->queries is absent or not an array, this notice is emitted:

perflab_register_default_server_timing_before_template_metrics(): Unable to compute server timing for "before-template-db-queries" because $wpdb->queries is not an array.

If a saved query array doesn't have the expected elapsed time as the second argument, then this notice is emitted:

perflab_register_default_server_timing_before_template_metrics(): Unable to compute server timing for "before-template-db-queries" because a saved query in $wpdb->queries lacks the elapsed time as the second array value.

Otherwise, if all is good, then the before-template-db-queries server timing entry is sent: