Add back asserts to catch unintentional use of generic SpanHelpers overloads

[jkotas]

1. Revert Partial Revert "Add asserts to catch unintentional use of generic overloads (#1200)" #1628
2. Add fixes as necessary to fix the discovered issues. Relax some asserts in SpanHelpers.T.cs #1472 has some of them.

[safern]

We now have the right coverage in PRs in order to be able to do this as #1473 is now merged.

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