Actor grouping

When the actors' hierarchy grows or there are short-living actors referenced by a unique path name, it can be impractical to give each metric a unique set of attributes. This amplifies the amount of data exported and can affect collector performance. To solve this, Mesmer allows the definition of fine-grained rules on how metrics associated with different actors can be grouped together.

There are three actor attributes grouping options:

group - the metrics, collected for all the actors matching the given path, share the actor_path attribute.
instance - the metrics, collected for actors matching this path, have a unique actor_path attribute.
disabled - the metrics, collected for all the actors matching the given path, do not get actor_path attribute.

By default the path grouping is disabled. You can change this by launching the Mesmer extension with -Dio.scalac.mesmer.actor.reporting-default=group configuration option. This will give all the actors' metrics the attribute actor_path="/".

Alternatively, it is possible to override a single path grouping strategy as following

java -javaagent:path/to/opentelemetry-javaagent.jar \
  -Dotel.javaagent.extensions=path/to/mesmer-otel-extension.jar \
  -Dio.scalac.mesmer.actor.reporting-default=ignore \
  -Dio.scalac.mesmer.actor.rules."/user/**"=group \
  -Dio.scalac.mesmer.actor.rules."/system/**"=group \
  -jar your-app.jar

This configuration will aggregate metrics for system and user hierarchies separately, grouping them by actor_path="/system" and actor_path="/user" attributes correspondingly.

The individual rule syntax is io.scalac.mesmer.actor.rules."<MATCHING_PATH>"=<GROUPING_OPTION>. The matching path supports limited set of wildcards, aiding to group metrics for variable path segments.

/ - matches exactly one actor and therefore can be used only with the instance grouping option. For a topology of /user/my-actor and /user/my-actor/1 and the rules
```
io.scalac.mesmer.actor.reporting-default=group
io.scalac.mesmer.actor.rules."/user/my-actor"=instance
```
the metrics from /user/my-actor will get the attribute actor_path="/user/my-actor" while the metrics from /user/my-actor/1 will get the attribute actor_path="/" and likely will be grouped with metrics from other actors.
/* - matches a single variable tailing segment. For a topology of /user/my-actor, /user/my-actor/1, /user/my-other-actor and the rules
```
io.scalac.mesmer.actor.reporting-default=ignore
io.scalac.mesmer.actor.rules."/user/*"=instance
```
the metrics from /user/my-actor and /user/my-other-actor will get same values for actor_path attribute, while /user/my-actor/1 will be ignored.
/** - matches all tailing segments. For a topology of /user/my-actor, /user/my-actor/1, /user/my-other-actor and the rules
```
io.scalac.mesmer.actor.reporting-default=ignore
io.scalac.mesmer.actor.rules."/user/**"=instance
```
this will produce a unique metric for each actor with root at /user. This wildcard also can be applied using with grouping,
```
io.scalac.mesmer.actor.reporting-default=ignore
io.scalac.mesmer.actor.rules."/user/**"=group
```
Resulting in all metrics from actors with root at /user to be aggregated using common attribute actor_path="/user"