metrics.base_metrics.Counter API

Is Counter meant to guarantee monotonically increasing values? It doesn't at the moment, since there's both a `dec` method and an `inc` method that accepts negative values.

What's the reason behind not providing a direct value setter? Even if we require the value to be monotonically increasing, you could for example check the current value and raise an exception if the new value is less. If there were a setter, then the `GarbageMetrics.collect` method in https://github.com/elastic/apm-agent-python/pull/481 could be simplified.