When putting new dynamic templates, all existing templates are overwritten.

[jtibshirani]

Elasticsearch version (bin/elasticsearch --version): 6.2.3

Description of the problem including expected versus actual behavior:

Putting new mappings for dynamic_template overwrites all existing templates, even templates that the put mappings request does not mention. This behavior conflicts with what's described in the dynamic templates documentation:

New templates can be appended to the end of the list with the PUT mapping API. If a new template has the same name as an existing template, it will replace the old version.

If we're happy with the behavior described in the documentation, we should update the mapping merge logic to obey those semantics.

Steps to reproduce:

Please see the following gist: https://gist.github.com/jtibshirani/52d913410957b502d6037f02d1f1340a

[elasticmachine]

Pinging @elastic/es-search-aggs

[jtibshirani]

It turns out this is a duplicate of #24853, where we decided to keep the current behavior. I opened #32890 to fix the documentation.