fix(tracing): eliminate aws service request time from latencies.kong

[windmgc]

Summary

As per FTI-5261's report, it is unexpected that the external request time of AWS lambda service is also being added to latencies.kong latency number, which is part of our serialized log data and being used by some of our monitoring plugins, such as datadog.

This PR tries to eliminate the AWS related request time from latencies.kong, by adding a new ctx variable ctx.KONG_THIRDPARTY_ACCESS_TIME. Since this problem is not a special one, we have a similar situation in many other plugins/vaults that requires external access(such as external IdP, secret management service, etc.). We can also add those things into ctx.KONG_THIRDPARTY_ACCESS_TIME so that we can tracking external 3rd-party service's latency, as well as subtracting them from Kong's latency number.

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

https://konghq.atlassian.net/browse/FTI-5261