Support observations in Spring Boot starter

[onobc]

Once observations are in the core for send/receive let's make it easier for end users to consume.

Easily disable/enable

• Add property to en/disable observations for send, receive, globally

Easier customization

• Make it easier for users to add custom tags. As it stands they would need to code up a convention and set it on the observed target. Let's at least create a convention customizer that can be auto-configured. NOTE: SB auto-configures observation registry and other components. Can we leverage this in any way?

Docs

• Make sure there is an easy to follow section in docs telling how end user can use this.
• Make sure there is an easy to follow section in docs telling how end user can access the built-in Pulsar consumer/producer stats.

Random cleanup

• Revisit the observation if/else blockage in the send/receive methods to clean it up possibly

Verify

• Test drive the above docs manually w/ Wavefront (or other platform). Ping @marcingrzejszczak when we do as he requested some screenshots in Add Observations for send/receive #146 (comment).