Add environment variables support for Client configuration

[clamoriniere]

Support of 3 environment variables for Client configuration:

• "DD_AGENT_HOST" used to provide the Agent hostname.
• "DD_DOGSTATSD_PORT" used to provide the DogStatsD port.
• "DD_ENTITY_ID" used to provide an identifier to the Client.

[lucaspimentel]

Left a few comments regarding the amount of (avoidable) null checks in the code.

[lucaspimentel]

The references to constantTags and tags are never set in this method, so there's no need for the ref keyword. Arrays are always passed by reference, if that was the intent.

[clamoriniere]

Ok, thank you for the info

[lucaspimentel]

I think it we can make the code around tags simpler by avoiding null arrays entirely. For example, early in the GetCommand() overloads, we can replace null arrays with empty ones.

// in Statsd class
private static readonly string[] EmptyArray = new string[0];

// in GetCommand()
constantTags = constantTags ?? EmptyArray;
tags = tags ?? EmptyArray;

If we guarantee that the constantTags and tags arrays are never null, we wouldn't need IsNullOrEmptyT<T>() and ConcatTags() becomes much simpler:

private static string ConcatTags(string[] constantTags, string[] tags)
{
    // concatenate all tags into a single enumerable
    var allTags = constantTags.Concat(tags);

    return "|#" + string.Join(",", allTags);
}

(edit: add a global empty array to avoid multiple allocations)

[clamoriniere]

I took your remarks, and update the PR.

[lucaspimentel]

We might want to use List<T> if we plan on resizing the collections. Then this code becomes simply:

_constantTags.Add(entityIDTag);

[clamoriniere]

done

[lucaspimentel]

Zero is more idiomatic as the "not set" value here, unless -1 is already defined elsewhere. Alternatively, we should check for both, or anything less than or equal to zero.

Suggested change

	if (Port == -1)
	if (Port == 0)

[clamoriniere]

ok

[clamoriniere]

@lucaspimentel Thanks the review.
I updated the PR with your comments.

[clamoriniere]

@lucaspimentel any plan for merging it?

[lucaspimentel]

@clamoriniere I'm not a maintainer on this repo or have write access. I think @yannmh can do it. Sorry about the confusion.

[lemkepf]

Can we get a nuget release for this? The ConstantTags would be HUGE.

[yannmh]

@lemkepf we are on it. ETA is before the week.