Avoid infinity loop when exporter use http to send spans

[OlivierAlbertini]

Is your feature request related to a problem? Please describe.

Right now, I must use this config to avoid infinity loop:

const config = { 
 plugins: {
            http: {
                enabled: true,
                path: '@opentelemetry/plugin-http',
                ignoreOutgoingUrls: [/spans/]
            }
        }
}

Describe the solution you'd like
We should handle this internally. Perhaps we could use a specific header that will be handled by http plugin for ignoring outgoing request with that header?

[vmarchaud]

We have a specific header with Opencensus to avoid this issue: https://github.com/census-instrumentation/opencensus-node/blob/master/packages/opencensus-instrumentation-http/src/http.ts#L46

[OlivierAlbertini]

should we set x-opentelemetry-outgoing-request ?

[vmarchaud]

@OlivierAlbertini i'm good with the header you set in the PR