fix: allow multiple instances of core to interact with context

[dyladan]

Because of the way node handles modules, multiple instances of core may exist in the same installation. For instance, core may be depended on by a propagator, a plugin, and the tracing module. If any of these depend on a slightly different version, the functions used to interact with context will fail to work together. Using Symbol.for ensures these symbols are the same regardless of the instance of core that is being used.

[dyladan]

Fixes #1379

[codecov]

Codecov Report

Merging #1387 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1387   +/-   ##
=======================================
  Coverage   94.03%   94.03%           
=======================================
  Files         149      149           
  Lines        4322     4322           
  Branches      880      880           
=======================================
  Hits         4064     4064           
  Misses        258      258           

Impacted Files	Coverage Δ
packages/opentelemetry-context-base/src/context.ts	77.77% <100.00%> (ø)

[naseemkullah]

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/for

In contrast to Symbol(), the Symbol.for() function creates a symbol available in a global symbol registry list. Symbol.for() does also not necessarily create a new symbol on every call, but checks first if a symbol with the given key is already present in the registry. In that case, that symbol is returned. If no symbol with the given key is found, Symbol.for() will create a new global symbol.