Sum metric agg

[DWalker487]

What does this do?

This adds support for the Elasticsearch Sum metric aggregator
https://www.elastic.co/guide/en/elasticsearch/reference/7.6/search-aggregations-metrics-sum-aggregation.html

Why is it a good idea?

This is one of the core elasticsearch metric aggregations, and is natively supported by Grafana.

Context

The need for this arose when wanting to combine multiple documents reporting at the same interval to generate a total. This metric can also be hidden, as per #231.

Questions

[dholbach]

Can you please resolve the conflict in CHANGELOG.rst?

[dholbach]

This seems to lead to #245 - do you think you can check?

[daniel@reef grafanalib ]$ python -c 'from grafanalib.elasticsearch import AverageMetricAgg; AverageMetricAgg().to_json_data()'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/daniel/dev/grafanalib/grafanalib/elasticsearch.py", line 78, in to_json_data
    'id': str(self.id),
AttributeError: 'AverageMetricAgg' object has no attribute 'id'
[daniel@reef grafanalib ]$ 

[DWalker487]

Ah yes - I missed an instance where I set up a default id when splitting out commits into different PRs - I'll update this now.