Resource improvement - Option 1

[CodeBlanch]

Fixes #1397.

Changes

Stores Resource on each Activity as a tag instead of as a custom property. This is to avoid an allocation on every Activity currently done to store Resource as a custom property.

This is by far the most expensive option from a performance perspective. We store Resource on every Activity. You have to enumerate on tags to retrieve it. And then you have to exclude the resource tag from being written out with other tags during export. Not a huge hit, but the other options avoid it all.

Also, every Exporter would have to know to exclude the special otel.resource tag. We could document to exclude all "otel.*" tags, but that carries an even more expensive comparison while looping, even bigger perf hit.

TODOs:

• CHANGELOG.md updated for non-trivial changes
• Design discussion issue Avoid Activity.SetCustomPropery usage for Resource. #1397
• Changes in public API reviewed

[cijothomas]

Only reason to consider option1, : If .NET 6 can support storing Resource in ActivitySource, then all the perf issues will be eliminated.
(Though there is still the "legacy" new Activity() which has default ActivitySource issue).

Lets keep this PR open for now, and see if Option3 can be merged. If Option3 works, then we are good!