fix: zipkin-exporter: don't export after shutdown

[Flarna]

Which problem is this PR solving?

According to spec and exporter should return FailedNotRetryable error
after shutdown was called.

see https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/sdk-tracing.md#shutdown-1

After the call to Shutdown subsequent calls to Export are not allowed and should return FailedNotRetryable error.

Short description of the changes

Remember that shutdown() was called and if yes immediate call callback with error.

[codecov-io]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.33%. Comparing base (b57951c) to head (6ae5d63).

Files with missing lines	Patch %	Lines
...ckages/opentelemetry-exporter-zipkin/src/zipkin.ts	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #526      +/-   ##
==========================================
- Coverage   92.40%   92.33%   -0.08%     
==========================================
  Files         146      146              
  Lines        7138     7080      -58     
  Branches      628      630       +2     
==========================================
- Hits         6596     6537      -59     
- Misses        542      543       +1     

Files with missing lines	Coverage Δ
.../opentelemetry-exporter-zipkin/test/zipkin.test.ts	100.00% <100.00%> (ø)
...ckages/opentelemetry-exporter-zipkin/src/zipkin.ts	98.43% <85.71%> (-1.57%)	⬇️

... and 10 files with indirect coverage changes

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[mayurkale22]

Looks like same thing needs to be implemented for Jaeger exporter.

[obecny]

lgtm, one comment

[Flarna]

Seems we have an issue with the just released node type definitions (DefinitelyTyped/DefinitelyTyped#40118):

src/http.ts:343:15 - error TS2345: Argument of type 'UrlWithStringQuery | null' is not assignable to parameter of type 'RequestOptions | (RequestOptions & Partial<UrlWithParsedQuery>) | null'.
  Type 'UrlWithStringQuery' is not assignable to type 'RequestOptions | (RequestOptions & Partial<UrlWithParsedQuery>) | null'.
    Type 'UrlWithStringQuery' is not assignable to type 'RequestOptions & Partial<UrlWithParsedQuery>'.
      Type 'UrlWithStringQuery' is not assignable to type 'RequestOptions'.
        Types of property 'protocol' are incompatible.
          Type 'string | null' is not assignable to type 'string | undefined'.
            Type 'null' is not assignable to type 'string | undefined'.