[APM] Number of environments displayed in the selector should be increased and made configurable

[ogupte]

Currently the number of environments loaded in the environment filter is limited to the default size of the terms agg when we query for them (https://github.com/elastic/kibana/blob/master/x-pack/plugins/apm/server/lib/ui_filters/get_environments.ts#L49). Ideally, we want to load all environments in the selector. One workaround now is to use the query bar service.environment : "test" but only if the target environment is known ahead of time.

One solution for this is to set a size on the terms agg that is sufficiently large for most use cases. Also we could make the size configurable. if we changed it to a composite aggregation, we can be sure to return all unique environments, but may have to deal with the complexity of paging results.

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[sorenlouv]

Closed in #82090