Modifying resources specification to describe capabilities (open-telemetry#196)

A focus of the specification on the capabilities that should be
afforded rather than strict requirements around implementation
enable better flexibility to ensure libraries are compatible with
the idioms of their corresponding languages.

This commit incorporates feedback discussed in open-telemetry#191.

Author: toumorokoshi

specification/api-resources.md

@@ -1,13 +1,13 @@
 # Resources API
 
-[Resource](../terminology.md#resources) represents the entity producing
+A [Resource](../terminology.md#resources) represents the entity producing
 telemetry. The primary purpose of resources as a first-class concept in the API
 is decoupling of discovery of resource information from exporters. This allows
 for independent development and easy customization for users that need to
-integrate with closed source environments. API MUST allow creation of
-`Resources` and associating them with telemetry.
+integrate with closed source environments. API MUST allow for creation of
+`Resources` and for associating them with telemetry.
 
-When used with distributed tracing resource can be associated with the
+When used with distributed tracing, a resource can be associated with the
 [Tracer](tracing-api.md#tracer) or individual
 [SpanData](tracing-api.md#spandata). When associated with `Tracer`, all `Span`s
 produced by this `Tracer` will automatically be associated with this `Resource`.
@@ -23,54 +23,61 @@ TODO: notes how Resources API is extended when using `SDK`. https://github.com/o
 
 ## Resource creation
 
-API defines two ways to instantiate an instance of `Resource`. First, by
-providing a collection of labels - method [Create](#create). The typical use of
-this method is by creating resource from the list of labels taken from
-environment variables, metadata API call response, or other sources.
+The API interface must support two ways to instantiate new resources. Those
+are:
 
-Second, by merging two `Resource`s into a new one. The method [Merge](#merge) is
-used when a `Resource` is constructed from multiple sources - metadata API call
-for the host and environment variables for the container. So a single `Resource`
-will contain labels for both.
+### Create
 
-Lastly, it is a good practice for API to allow easy creation of a default or
-empty `Resource`.
+The interface MUST provide a way to create a new resource, from a collection 
+of labels. Examples include a factory method or a constructor for 
+a resource object. A factory method is recommended to enable support for 
+cached objects.
 
-Note that the OpenTelemetry project documents certain ["standard
-labels"](../semantic-conventions.md) that have prescribed semantic meanings.
+Required parameters:
 
-### Create
+- a collection of name/value labels, where name and value are both strings.
 
-Creates a new `Resource` out of the collection of labels. This is a static
-method.
+### Merge
 
-Required parameter:
+The interface MUST provide a way for a primary resource to merge with a 
+secondary resource, resulting in the creation of a brand new resource. The 
+original resources should be unmodified.
 
-Collection of name/value labels where both name and value are strings.
+This is utilized for merging of resources whose labels come from different
+sources, such as environment variables, or metadata extracted from the host or 
+container.
 
-### Merge
+Already set labels MUST NOT be overwritten unless they are the empty string. 
 
-Creates a new `Resource` that is a combination of labels of two `Resource`s. For
-example, from two `Resource`s - one representing the host and one representing a
-container, resulting `Resource` will describe both.
+Label key namespacing SHOULD be used to prevent collisions across different 
+resource detection steps.
 
-Already set labels MUST NOT be overwritten unless they are empty string. Label
-key namespacing SHOULD be used to prevent collisions across different resource
-detection steps.
+Required parameters:
 
+- the primary resource whose labels takes precedence.
+- the secondary resource whose labels will be merged.
 
-Required parameter:
+### The empty resource
 
-`Resource` to merge into the one, on which the method was called.
+It is recommended, but not required, to provide a way to quickly create an empty 
+resource.
+
+Note that the OpenTelemetry project documents certain ["standard
+labels"](../semantic-conventions.md) that have prescribed semantic meanings.
 
 ## Resource operations
 
-API defines a resource class with a single getter for the list of labels
-associated with this resource. `Resource` object should be assumed immutable.
+In addition to resource creation, the following operations should be provided:
+
+### Retrieve labels
+
+The API should provide a way to retrieve a read only collection of labels
+associated with a resource. The labels should consist of the name and values, 
+both of which should be strings.
 
-### GetLabels
+There is no need to guarantee the order of the labels.
 
-Returns the read only collection of labels associated with this resource. Each
-label is a string to string name value pair. The order of labels in collection
-is not significant. The typical use of labels collection is enumeration so the
-fast access to the label value by it's key is not a requirement.
+The most common operation when retrieving labels is to enumerate over them.
+As such, it is recommended to optimize the resulting collection for fast 
+enumeration over other considerations such as a way to quickly retrieve a
+value for a label with a specific key.