Add ELB section for lambda instrumentation

[basepi]

Elastic Load Balancer (ELB) can now be attached to lambda functions directly, without an API Gateway. This necessitates an additional section for event parsing.

There is no obvious candidate for trigger.request_id, so I've omitted it for this trigger type.

I've also removed the context.service.origin.version as compared to API gateway, since it's not relevant for ELB.

Python implementation here

• [x] Create PR as draft
• [x] Approval by at least one other agent
• [x] Mark as Ready for Review (automatically requests reviews from all agents and PM via CODEOWNERS)
  • Remove PM from reviewers if impact on product is negligible
  • Remove agents from reviewers if the change is not relevant for them
• [x] Approved by at least 2 agents + PM (if relevant)
• [x] Merge after 7 days passed without objections
• [x] Create implementation issues through the meta issue template (this will automate issue creation for individual agents)

/schedule 2022-08-30

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#### Build stats * Start Time: 2022-08-29T05:27:00.604+0000 * Duration: 5 min 49 sec

[basepi]

Scheduled to be merged on 2022-08-30