[SCHEMA] The MetricExporter schema is unsafe

[marcalff]

Fixes #109

Changes

Split MetricExporter into:

• PushMetricExporter, which can represent otlp, console, or an extension point,
• PullMetricExporter, which can represent prometheus, or an extension point,

This is to enforce that:

• PeriodicMetricReader can only use a PushMetricExporter
• PullMetricReader can only use a PullMetricExporter

[marcalff]

Note that this is a blind push.

I could not get instructions to work to run tests:

[malff@malff-desktop opentelemetry-configuration]$ npm --version
6.14.11
[malff@malff-desktop opentelemetry-configuration]$ make
npm install
npm WARN opentelemetry-configuration No repository field.
npm WARN opentelemetry-configuration No license field.

audited 110 packages in 0.598s

57 packages are looking for funding
  run `npm fund` for details

found 1 high severity vulnerability
  run `npm audit fix` to fix them, or `npm audit` for details
All tools installed
/data/malff/CODE/PLAY_GITHUB/opentelemetry-configuration
└── [email protected] 

npx: installed 1 in 0.487s
command not found: compile
make: *** [Makefile:11: compile-schema] Error 1

[jack-berg]

Not sure what you're node error is all about. I use node version 18 which comes with npm version 9. Maybe we should check an .nvmrc into version control.