🐛[RUMF-639] xhr proxy: do not instrument xhr already opened

[bcaudan]

Motivation

Issue raised by customer

Changes

Check that xhr has been instrumented on open to instrument send

Testing

Automated tests

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I have gone over the contributing documentation.

[codecov-commenter]

Codecov Report

Merging #484 into master will decrease coverage by 0.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #484      +/-   ##
==========================================
- Coverage   87.73%   87.60%   -0.14%     
==========================================
  Files          34       34              
  Lines        2112     2113       +1     
  Branches      413      414       +1     
==========================================
- Hits         1853     1851       -2     
- Misses        259      262       +3     

Impacted Files	Coverage Δ
packages/core/src/xhrProxy.ts	100.00% <100.00%> (ø)
packages/rum/src/parentContexts.ts	98.21% <0.00%> (-1.79%)	⬇️
packages/rum/src/rum.ts	89.23% <0.00%> (-1.54%)	⬇️

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