A basic test to verify the converted bug in handling tags.

Refactor to allow running the same test logic with different inputs and
expected results.

Add an e2e test for global labels values with multiple events.

A batch of 2 events in input with different labels Values but
same key should produce in output events with all different
Values added to the same key.

Fix unrelated typos found while exploring the code.

We only retain the latest tags in a batch of events.

TestAggregateAndHarvest sort logic only used Metricset.Name.
The latest test added to it s flappy with such a logic, as we are
evaluating the same Metricset.Names in 2 different events with
global labels with equal key and different values.

Enhance the sorting logic to account for equal Metricset.Name and
leverage labels (keys, Value and Values) in the lessFn.

When 2 events in a batch have the same service name but
different tags we output 2 different sets of metrics,
one for each tag set. This was not considered in the
previous expected values.

This reverts commit 49d0906.

The same test is covered by TestMarshalEventGlobalLabelsRace,
which also test for a possible race condition.

There was a typo, this loop should have iterated over b.Labels
but was iterating over a.Labels

The previous implementation could panic if len(b) > len(a). Add
a guard clause that limit iterating over label based on len(b).

If len(a.Labels) > len(b.Labels), everything else equal we
conclude that a > b.

Leverage a string comparison as a sort function, as
used in https://github.com/elastic/apm-data/blob/81d77648a3b17d4e52859110e233e40683fcdfe3/input/otlp/metrics_test.go#L1146-L1148

Adds an additional tag to the test case to ensure
expected results.