ElasticMeterRegistry should allow indexType override

[magaton]

Currently, for ES versions < 7, indexType is hardcoded in TYPE_PATH_BEFORE_VERSION_7 static variable.
It would be good to have a way to override the indexType, so it can be something other than 'doc'

[shakuzen]

As a note for future planning, once we no longer support Elasticsearch versions less than 7, we should remove this configuration option.

Based on Elasticsearch's published EOL schedule, it looks like that will be sometime around the end of this year: https://www.elastic.co/support/eol

Elasticsearch	EOL date	Maintained until
6.8.x	2020-11-20	8.0.0

[shakuzen]

I think we made a mistake when we made this a method on ElasticMeterRegistry instead of a method in ElasticConfig and use that in ElasticMeterRegistry. I'd like to change that before releasing 1.4, so I am reopening this. In fact, we had a change for that already (#1328) that I missed when reviewing this issue and corresponding pull request.