profiler: logically cluster the Config fields
profiler: logically cluster the Config fields

Also improve the godoc for some fields by explaining
what "testing" env means.

Change-Id: I3ce9b52bb4d215796a9db03f67a201e266cfecd0
Reviewed-on: https://code-review.googlesource.com/14511
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Reviewed-by: Jianqiao Li <2d515f767da37bdb8a87cabf23e3e650ab8dc69c@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go