tracer/option: Add envvar DD_CLIENT_HOSTNAME_ENABLED to config client hostname detection

[ajgajg1134]

What does this PR do?

Adds env var "DD_CLIENT_HOSTNAME_ENABLED" which allows users to disable client hostname detection by setting this value to false (Note the value defaults to true)

Motivation

Some customers may need to disable this if they have aggressive security tooling that alerts on access to cloud metadata APIs.

Describe how to test/QA your changes

unit tests are good!

Reviewer's Checklist

• Changed code has unit tests for its functionality.
• If this interacts with the agent in a new way, a system test has been added.

[pr-commenter]

Benchmarks

Comparing candidate commit ea6420b in PR branch andrew.glaude/AllowDisableClientHost with baseline commit 78e9722 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 18 metrics, 0 unstable metrics.

[ahmed-mez]

Any thoughts on using the option disableHostnameDetection that already exists here?
It's a bit confusing that the new env var DD_DISABLE_CLIENT_HOSTNAME doesn't directly control the boolean option disableHostnameDetection.
Most tracer config env vars are read in https://github.com/DataDog/dd-trace-go/blob/main/ddtrace/tracer/option.go and the config defaulting happens there, so by having the new env var read and applied there we keep the logic consolidated in one function newConfig which improves readability.
I understand that you might have added the env var check in hostname.Get() to cover the initial fill done in tracer.Start which was a nice catch but there are other options to handle it with the new suggestion

• Move the initial fill to tracer/option.go, and only do it if config.disableHostnameDetection is false after defaulting.
• The initial fill stays in tracer.Start but done via _ = t.hostname() instead of hostname.Get() to make sure config.disableHostnameDetection is checked.

Apart from that, I wanted to re-suggest a comment from the original PR about using enableHostnameDetection instead of disableHostnameDetection (also applies to the env var name) #1712 (comment) as I do believe it'd improve readability + less error prone, we'd get rid of double negatives like

dd-trace-go/ddtrace/tracer/tracer.go

Line 630 in 3ccf65d

if !t.config.disableHostnameDetection {

which actually caused of a bug that was caught in that same suggestion in the original PR.

[ahmed-mez]

Looks great, thanks! One final suggestion to add unit test. LGMT otherwise!

[ahmed-mez]

💯