Micro services should be following 12 Factor App principles and reading their configuration from environment variables.
Teams may choose to use either Spring or the Companies House environment-reader-library library. They should be consistent in their choice for all the micro services that form a coherent group.
It is recommended that Spring Boot projects use application.properties to refer to environmental variables so they can be used by Spring features such as SPEL expressions.
How to use Spring application configuration is provide in the Spring documention for external configuration