GH-2289: Add sub status to CMP fetch URL

[leuryr]

Adds the ss param to the CMP fetch URL. Feeling a bit conflicted about having the logic for this inside the _buildUrl method, though it is technically part of building the URL. Open to moving it into a separate function.

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[wlycdgr]

Looks like it works as expected, but I share your mixed feelings about having the logic be inside the _buildURL method. I suggest factoring it out to a static method of either the CMP or the Metrics class. While you're at it, could you convert CMP#_buildURL to use the _buildQueryPair method you made in Metrics? It can be (and should be, since it can be) converted to a static method, I think.

Probably most importantly, we should see if we can rename the key from ss to something else, to avoid the confusion of a key that means one thing in the context of a call to the CMP server and something else entirely in the context of a call to the metrics server. Let's work with JJ to see how we can do this - technically, it should be an easy change on the CMP server side.

[wlycdgr]

See comment (but disregard the part about the parameter naming)

[leuryr]

As we discussed, I moved the query string building to a util function that we can use for both the CMP & Metrics classes (with a small addition for when we need a ? at the beginning of the returned string instead of &), and I factored getting the sub status out from CMP_buildURL into its own static method.

[wlycdgr]

LGTM