New replace_keys Remap function

[bruceg]

Blocked in #5785. The remap language has support for manipulating strings in event values, for example with the replace function. To do the equivalent with keys is not possible.

The use case is described in #3588, where keys containing dots can lead to broken indexing. A trivial solution would be to replace all dots with underscores, as is done by the de_dot filter in both Elasticsearch and Logstash. Better would be to support fully parameterized replacement just like the replace function does for values.

Suggested function: replace_keys(map, regex | string, string, count = integer, depth = boolean) :: map
Parameters are the same as for replace except for the first and the return type, both of which are maps instead of strings, and the additional depth options, which will cause it to recurse into maps and replace all their keys as well.

Usage for the scenario above would be something like:
. = replace_keys(., ".", "_", depth = true)

[jszwedko]

Related: #4518

[binarylogic]

I'm curious if we want to accept a value as the first argument here? Whenever we use:

. = ...

It is a flag to me that the function should operate on the entire event (like del). Otherwise, you could make the case that we should do:

. = del(.key)

As a first step, I think it's simpler to just make this an event-level function:

replace_keys('.', '_')

That example would replace all keys on the event.

[JeanMertz]

Closing this, as we now have support for map_keys, which works similarly, but is more flexible:

 . = map_keys(object!(.), recursive: true) -> |key| { replace(key, ".", "_") }