Fix varmap to apply func before recursion

[basepi]

What does this pull request do?

We use varmap() to apply shorten() recursively across a data structure. varmap() was recursing into the data structures, and then applying the func() (in this case, shorten()) on the way back out of recursion. This meant that when shortening local variables where just recursing over the data structure was a performance issue, setting ELASTIC_APM_LOCAL_VAR_LIST_MAX_LENGTH wouldn't actually affect the runtime, just the output.

With this change, dictionaries and lists will be shortened and then we'll iterate over their contents. Shouldn't affect _sanitize() at all, but will speed up shorten() considerably.

I think the test surface on this function is sufficient without adding tests for this fix.

Related issues

Closes #1592

[ghost]

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• Start Time: 2022-07-21T23:16:47.661+0000

• Duration: 26 min 44 sec

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Total	8377

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Name	Metrics % (covered/total)	Diff
Packages	100.0% (68/68)	💚
Files	100.0% (227/227)	💚
Classes	100.0% (227/227)	💚
Lines	89.642% (17257/19251)	👍 0.022
Conditionals	76.869% (3147/4094)	👍 0.098

[beniwohli]

Great catch! LGTM