Add support to assign dynamic tag into existing metric

From slack thread it would be nice to have ability to define only dynamic part in metric while metric with static tags can be defined only once:

registry.timer("my.timer", "static", "tag", "response", response.status()).record(1);

->

Timer myTimer = registry.timer("my.timer", "static", "tag")
...
myTimer.withTags("response", response.status()).record(1);

where withTags returns new instance of timer with additional tags attached.

[jkschneider]

@zeratul021 It is possible to use dynamic tags now though, simply by registering it as you use it. If the meter already exists, Micrometer will return the existing one.

[hanagiat]

https://dzone.com/articles/spring-boot-metrics-with-dynamic-tag-values - link to the source inside the article