Add tracer construction spec

specification/tracer-construction.md

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+# OpenTelemetry tracer construction and resolution
+
+This document describes tracer object construction and how it is provided to the user application.
+
+User code should not directly instantiate the tracer and it MUST use `OpenTelemetry.getTracer()`
+to access the tracer. Behind the scenes `OpenTelemetry` uses `TracerProvider` which returns
+a configured tracer instance. `TracerProvider` is part of the trace implementation (or runtime) and 
+it is registered to `OpenTelemetry`.
+
+The tracer object construction depends on the implementation. The implementation might require
+specifying various configuration properties at creation time. Therefore to make it work with
+parameterless `OpenTelemetry` the configuration should be done via external configuration files.
+
+## Tracer provider
+
+Tracer provider is an internal class used by `OpenTelemetry` to get a tracer instance.
+Implementations MUST provide an implementation `TracerProvider` which is used by API package to get
+a tracer instance. The tracer provider is registered to API usually via language-specific mechanism.
+For instance in Java `ServiceLoader` is used.
+
+### Runtime with multiple deployments/applications
+
+An application runtime might provide its own implementation of `TracerProvider` with higher
+loading priority than the provider packaged inside the implementation. This provider can return
+a different tracer instance per deployment/application.