Use more structured metrics approach based on Micrometer

[gr-hovest-atb]

After updating to Spring Boot 2, we are using the new MeterRegistry for counting calls in the gateway.

This can be used as starting point for major improvements:

• Instead of just creating a long string as "counter ID", we could use a more structured approach using tags/dimensions.
• Instead of manually transforming Spring metrics to CoatRack metrics in our gateway and write our own transmission code, we could use one of the supported monitoring systems to let the gateway push the metrics to: https://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#actuator.metrics
• Analysis of the metrics on CoatRack admin side could be based on the monitoring systems rather than our own metrics DB
• Also performance of the gateway could be monitored this way

To be further discussed. This should be seen as a super-issue, while concrete development steps should be defined as smaller sub-issues.

[ChristophBaierATB]

Currently, the API key value is stored in the metrics ID string which represents a severe security problem. A possible solution would be to store the API key values as (salted) hashed values.

[ChristophBaierATB]

Currently, the our newly added transmission of metrics (see #152) requires is synchronous with its receival. This could lead to performance problems.

• [ ] Make sure that our new metrics approach uses asynchronous data transmission.