add code that seems to be needed for instrumentation

[jefimm]

From reading the https://github.com/DataDog/dd-trace-java/tree/master/docs and examining the existing instrumentations it seems that this should be the code requried for the instrumentaiton support

#517

[ptaoussanis]

@jefimm Hi Yefim, thanks for this.

To help make things easier to review for maintainers that may have limited time and/or context, could you please share some more information?

For example-

• How does DataDog fit into this?
• You've linked to some long-form documentation there; can you please pinpoint what precisely is relevant, and explain how it's motivating the current PR?
• How precisely does this relate to Elastic APM Agent distributed tracing support #517, and @bsless's suggestion there. Does this fully address Elastic APM Agent distributed tracing support #517? Are there trade-offs or alternatives that still need to be weighed? Etc.

The easier you can make a PR to review, the more likely they'll be to be reviewed in good time.

Much appreciated!