chore(grpc-instrumentation): fix grpc example #2160

[vmarchaud]

There were two problems that i've fixed:

• sometime a client method (that is generated by grpc client) do not have a path, as i don't know the grpc internal i'm don't know why
• there was a race between instrumentation files patch and the global one, which sometimes would not set the grpcClient variable (which is used to retrieve grpc constants).

Fxies #2160

[codecov]

Codecov Report

Merging #2179 (752f68a) into main (cd54a45) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2179      +/-   ##
==========================================
+ Coverage   92.80%   92.82%   +0.01%     
==========================================
  Files         140      140              
  Lines        5002     5002              
  Branches     1029     1030       +1     
==========================================
+ Hits         4642     4643       +1     
+ Misses        360      359       -1     

Impacted Files	Coverage Δ
...entelemetry-instrumentation-grpc/src/grpc/index.ts	93.04% <100.00%> (ø)
...emetry-core/src/platform/node/RandomIdGenerator.ts	93.75% <0.00%> (+6.25%)	⬆️