Document AnyValue attribute limits

[lmolkova]

#4485 suggests using the following limits for complex attribute values:

Attribute value length limit SHOULD be applied to all leaf string nodes in
AnyValue which SHOULD be truncated to the configured limit.

Leaf nodes of an AnyValue attribute SHOULD count toward the attribute count limit.
If the limit is reached, the SDK MUST drop the entire AnyValue attribute;
partial exports are not allowed.

As well as potential ability on AnyValue to estimate its total size (which can be done at the construction time for performance reasons).

We can consider a separate set of attribute limits specifically for complex values, including a total size limit and ability to estimate AnyValue object size.

It's related to #4006 - a similar set of limits would be applicable to the log body with a caveat that log bodies may need a separate limit since log bodies are expected to be larger than attributes and backends tolerate large log bodies much better.

Regardless of the #4485 outcome, we need to define limits at least for the log attributes.