Consider flattening object-valued tags

[djluck]

Package

None

Is your feature request related to a problem?

When attaching object-valued tag values to Activity, when they are exported via TagWriter they are converted to their string representation via Convert.ToString.

This leads to issues with the exported attribute values, where one of two things happens:

1. The object structure and values are not exported. This is the situation with the default object.ToString implementation.
2. The object structure is not exported. This is the situation when ToString has been overridden and while the object property names + values are visible, it is not efficient to query.

What is the expected behavior?

If we could flatten objects automatically into key/ value pairs, e.g. given an object structure like this:

public record MyRecord(string Foo, string Bar);

When including this as a tag:

using var activity = ActivitySource.StartActivity("my_activity").AddTag("myrecord", new MyRecord("hello", "sir"))

The "myreord" tag would be converted into multiple OTel attributes that followed OTel attribute naming conventions (i.e snake_case + lowercased):

myrecord.foo = hello
myrecord.bar = sir

As per the discussion in the specification repo around supporting maps, we wouldn't be able to serialize any non-primitive arrays.

Which alternative solutions or features have you considered?

Currently no alternative exists in the standard library. The presence of Activity.AddTag(string key, object value) means it's very likely that users will incorrectly add

Additional context

No response