chore(instr-express): add semconv to readme

[JamieDanielson]

Which problem is this PR solving?

• Updates Document instrumentation semantic conventions #1778

Short description of the changes

• In README, add version of Semantic Conventions used in express instrumentation. See more details re: version in the issue
• In README, add semantic convention attributes collected

[JamieDanielson]

As mentioned on the issue, I'm hoping to get feedback on this to see if it satisfies the goals of #1778 or if there are pieces to this that are missing. For example, I'm not sure if attributes listed are intended to just be those from semantic conventions, or if all attributes on an instrumented Express span should be listed here.

[codecov]

Codecov Report

Merging #1907 (a7fdfa6) into main (e29b72a) will increase coverage by 0.01%.
The diff coverage is n/a.

❗ Current head a7fdfa6 differs from pull request most recent head a1520f0. Consider uploading reports for the commit a1520f0 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1907      +/-   ##
==========================================
+ Coverage   91.02%   91.04%   +0.01%     
==========================================
  Files         146      147       +1     
  Lines        7491     7530      +39     
  Branches     1501     1507       +6     
==========================================
+ Hits         6819     6856      +37     
- Misses        672      674       +2     

see 1 file with indirect coverage changes

[pichlermarc]

As mentioned on the issue, I'm hoping to get feedback on this to see if it satisfies the goals of #1778 or if there are pieces to this that are missing. For example, I'm not sure if attributes listed are intended to just be those from semantic conventions, or if all attributes on an instrumented Express span should be listed here.

I think the goal there is to just to the actual semconv ones, so this PR satisfies the goals of that issue 🙂