key_value_buffer is static and would cause issues if two updaters are created

As the title says, in micro_ros_diagnostic_updater the buffer utilized for the updater is static, perhaps should be part of one of the internal structs utilized instead.

Locate the micro_ros_diagnostic_updater implementation and inspect how key_value_buffer is declared and used. Exercise the updater with two instances to reproduce shared-buffer interference, then verify that each updater keeps its own buffer without affecting the other.

Written by the indexing model from the issue text.