Change environment prefix from nms to nginx_agent

[spencerugbo]

Proposed changes

Agent now uses NGINX_AGENT as its environment prefix as opposed to NMS. If the old prefix is used, Agent migrates the value to the correct key and a message is logged warning the user to switch to the new prefix

Checklist

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• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• If applicable, I have added tests that prove my fix is effective or that my feature works
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• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13

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[olli-holmala]

nit: I think OldEnvPrefix should rather be called LegacyEnvPrefix and NewEnvPrefix just EnvPrefix. That way we could eventually get rid of LegacyEnvPrefix and we'd be left with EnvPrefix.

[olli-holmala]

What about the (admittedly unlikely) case where both oldKey and newKey are set?

[spencerugbo]

If both oldKey and newKey are set we assume the newKey is the one with the correct value and ignore the value from the old key

[olli-holmala]

Could you add some unit tests for this? I think Go's testing.T should support setting env vars nicely with t.Setenv(), so this should be testable.

[olli-holmala]

Generally it's good practice to use fmt.Sprintf() for building strings instead of "string" + "another_string". However, I think your solution is more readable so it's fine here.

I would perhaps reuse theagent_config.KeyDelimiter variable we have here:

Suggested change

	oldKey := strings.ToUpper(OldEnvPrefix + "_" + strings.ReplaceAll(flag.Name, "-", "_"))
	newKey := strings.ToUpper(NewEnvPrefix + "_" + strings.ReplaceAll(flag.Name, "-", "_"))
	oldKey := strings.ToUpper(
	OldEnvPrefix + agent_config.KeyDelimiter + strings.ReplaceAll(flag.Name, "-", agent_config.KeyDelimiter)
	)
	newKey := strings.ToUpper(
	NewEnvPrefix + agent_config.KeyDelimiter + strings.ReplaceAll(flag.Name, "-", agent_config.KeyDelimiter)
	)

[aphralG]

Will create task to check MigratedEnv in UnitTests in the future, otherwise LGTM