Update peer.* namespace to account for service.namespace

[mmanciop]

This PR renames peer.service to peer.service.name and introduces peer.service.namespace with the intention of supporting match the usage of service.namespace + service.name as identity for services.

This PR partly fulfils #2338

Changes

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• CONTRIBUTING.md guidelines followed.
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[mmanciop]

Mmm on second thought, I am starting to see too many dependencies to peer.service in various implementations:

• https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/05715e325ae5b138e4f00c21044cb6c2224c5637/pkg/datadog/config/traces.go#L50
• https://github.com/open-telemetry/opentelemetry-specification/blob/3fb5df3ec30467907c7295860ba08e5726375696/specification/trace/sdk_exporters/zipkin.md?plain=1#L94

Maybe a better option would be to add a peer.service_namespace attribute instead, and forsake symmetry with the service namespace?