docs: add usage for mongodb-core plugin #543

[vmarchaud]

I just added quick explaination based on the usage of the http plugin

[codecov-io]

Codecov Report

Merging #564 into master will increase coverage by 5.47%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #564      +/-   ##
==========================================
+ Coverage    87.1%   92.57%   +5.47%     
==========================================
  Files         145      158      +13     
  Lines        7437     8042     +605     
  Branches      663      674      +11     
==========================================
+ Hits         6478     7445     +967     
+ Misses        959      597     -362

Impacted Files	Coverage Δ
.../opentelemetry-plugin-redis/test/assertionUtils.ts	95.83% <0%> (-4.17%)	⬇️
...entelemetry-exporter-jaeger/test/transform.test.ts	100% <0%> (ø)	⬆️
packages/opentelemetry-tracing/src/config.ts	100% <0%> (ø)	⬆️
packages/opentelemetry-tracing/src/utility.ts	100% <0%> (ø)	⬆️
...telemetry-plugin-grpc/test/utils/assertionUtils.ts	100% <0%> (ø)	⬆️
packages/opentelemetry-tracing/test/Span.test.ts	100% <0%> (ø)	⬆️
...ry-tracing/test/export/SimpleSpanProcessor.test.ts	100% <0%> (ø)	⬆️
.../opentelemetry-core/src/trace/spancontext-utils.ts	100% <0%> (ø)	⬆️
...pe-async-hooks/test/AsyncHooksScopeManager.test.ts	100% <0%> (ø)	⬆️
.../opentelemetry-exporter-jaeger/test/jaeger.test.ts	100% <0%> (ø)	⬆️
... and 37 more

[obecny]

lgtm, one comment