log: Implement Stringer for Value and KeyValue

[pellared]

The failure messages from assertions like this one (I changed the test to fail) convinced me to implement Stringer.

        	Error Trace:	/home/rpajak/repos/opentelemetry-go/log/keyvalue_test.go:81
        	Error:      	Not equal: 
        	            	expected: false
        	            	actual  : true
        	Test:       	TestValueEqual
        	Messages:   	{[] 2 0xc0001a4410}.Equal({[] 2 0xc0001a4410})

Reference: #4809 (comment)

Originally posted by @pellared in #4949 (comment)

[pellared]

I would like to have consensus if we want to have it in initial stable release.

@MrAlias, do I remember correctly that you suggested removing this issue from the Logs RC project as it is NOT required by the OTel spec (it is only a convenience feature)? If so I am even leaning to closing this issue.

[MrAlias]

I'm not opposed to adding this. There are two issues though:

• The attribute package has a lot more configuration for string output with its encoders. Are we going to hit an issue if we make a single choice for how KeyValue are printed here?
• Will the comparable string be used as a comparison for equality checks or maps? Whether we want to or now users would then have a way to compare two Value outside of Equal. If we want this an encoder might be a better approach.

[pellared]

The `attribute package has a lot more configuration for string output with its encoders. [...] If we want this an encoder might be a better approach.

Cannot the user create its own encoding (and even decoding) API that takes log.Value and log.KeyValue? What is the use case of allowing a user-defined Encoder? Is there a place in e.g. Go standard library that allows custom encoding? I think that we can provide our opinionated encoding like e.g. https://pkg.go.dev/time.

Whether we want to or now users would then have a way to compare two Value outside of Equal.

I think it would be a user error.

We could add a doc comment like (inspired by https://pkg.go.dev/time#Time.String):

The returned string is meant for debugging; the representation is not stable.