[exporter/elasticsearch] [chore] Drop strftime dependency

[rockdaboot]

Description

Switch to Go style time format strings:

• drop strftime dependency
• simplify code

[carsonip]

This looks like a breaking change IIUC. It is both changing the default, and the expected format of the input config.

date_format(default=%Y.%m.%d): Time format (based on strftime) to generate the second part of the Index name.

Do you expect an existing config file with date_format: %Y.%m.%d to work after this change?

[rockdaboot]

Do you expect an existing config file with date_format: %Y.%m.%d to work after this change?

Oh I see 😅. I didn't follow the code-as-string pattern which leads to date_format as part of logstash_format, which is documented in README.md. My lack of understanding the higher level concepts here.

Does it make sense to add prefix_separator and date_format to testdata/config.yaml to make it more obvious that these fields are user-facing?