Improve MDC and Key-Value handling for labels/metadata

[nehaev]

What can be improved:

1. All values in MDC become labels/metadata
2. All Key-Value elements become separate labels/metadata
3. A particular element from Key-Value becomes a label/metadata
4. Exclusion list
5. Nullability, e.g. absent or null-valued MDC/Key-Value keys should be excluded from labels/metadata (refs Omit Null values from Structured Metadata #282)

Tech note: this might require a custom converter or even a parser/tokenizer.

[nehaev]

The main problem is that the current syntax for labels/metadata has only static keys:

app = my-app,
host = ${HOSTNAME},
level = %level,

Potential syntax for mdc and kvp could look like:

app = my-app
host = ${HOSTNAME}
level = %level
* = %%mdc                   // all MDC keys become labels/metadata
* = %%mdc{key1, key2}       // only MDC keys `key1` and `key2` become labels/metadata
* != %%mdc{key1, key2}      // all MDC keys excluding `key1` and `key2` become labels/metadata
mdc_* = %%mdc               // all MDC keys become labels/metadata with prefix `mdc_`

This option looks good. * represents a dynamic set of keys.

But comma no longer can serve as a separator between key-value pairs. Replacing it with a new line makes sense, but this will require a major release, e.g., v.2.0.0. Maybe it's a chance to revisit some other parts of the current config structure.

Also, it's under the assumption that %% does not conflict with anything in Logback.