Extend attribute naming rules to metric labels (open-telemetry#821)

* Extend attribute naming rules to metric labels

We earlier defined naming rules for attributes, however we do not have
similar rules for metric labels. This commit extends the exact same
set of rules to metric labels.

This was brought up in this comment open-telemetry/opentelemetry-specification#807 (comment)

* Address PR comments

specification/common/common.md

@@ -6,7 +6,7 @@ Table of Contents
 </summary>
 
 - [Attributes](#attributes)
-  - [Attribute Naming](#attribute-naming)
+  - [Attribute and Label Naming](#attribute-and-label-naming)
 </details>
 
 ## Attributes
@@ -37,12 +37,22 @@ indices that are kept in sync (e.g., two attributes `header_keys` and `header_va
 both containing an array of strings to represent a mapping
 `header_keys[i] -> header_values[i]`).
 
-### Attribute Naming
+## Attribute and Label Naming
 
-Attribute name (also known as the "attribute key") MUST be a valid Unicode
-sequence.
+_This section applies to Resource, Span and Log attribute names (also known as
+the "attribute keys") and to keys of Metric labels. For brevity within this
+section when we use the term "name" without an adjective it is implied to mean
+"attribute name or metric label key"._
 
-Attribute names SHOULD follow these rules:
+Every name MUST be a valid Unicode sequence.
+
+_Note: we merely require that the names are represented as Unicode sequences.
+This specification does not define how exactly the Unicode sequences are
+encoded. The encoding can vary from one programming language to another and from
+one wire format to another. Use the idiomatic way to represent Unicode in the
+particular programming language or wire format._
+
+Names SHOULD follow these rules:
 
 - Use namespacing to avoid name clashes. Delimit the namespaces using a dot
   character. For example `service.version` denotes the service version where
@@ -56,68 +66,67 @@ Attribute names SHOULD follow these rules:
   words by underscores (i.e. use snake_case). For example `http.status_code`
   denotes the status code in the http namespace.
 
-- Attribute names SHOULD NOT coincide with namespaces. For example if
+- Names SHOULD NOT coincide with namespaces. For example if
   `service.instance.id` is an attribute name then it is no longer valid to have
   an attribute named `service.instance` because `service.instance` is already a
-  namespace. Because of this rule be careful when choosing attribute names:
-  every existing attribute name prohibits existence of an equally named
-  namespace in the future, and vice versa: any existing namespace prohibits
-  existence of an equally named attribute in the future.
+  namespace. Because of this rule be careful when choosing names: every existing
+  name prohibits existence of an equally named namespace in the future, and vice
+  versa: any existing namespace prohibits existence of an equally named
+  attribute or label key in the future.
 
-#### Recommendations for OpenTelemetry Authors
+### Recommendations for OpenTelemetry Authors
 
-- All attribute names that are part of OpenTelemetry semantic conventions
-  SHOULD be part of a namespace. 
+- All names that are part of OpenTelemetry semantic conventions SHOULD be part
+  of a namespace.
 
-- When coming up with a new convention make sure to check existing namespaces
-  for
+- When coming up with a new semantic convention make sure to check existing
+  namespaces for
   [Resources](https://github.com/open-telemetry/opentelemetry-specification/tree/master/specification/resource/semantic_conventions),
   [Spans](https://github.com/open-telemetry/opentelemetry-specification/tree/master/specification/trace/semantic_conventions),
   and
   [Metrics](https://github.com/open-telemetry/opentelemetry-specification/tree/master/specification/metrics/semantic_conventions)
-  to see if the new attributes fits.
+  to see if the new name fits.
 
 - When a new namespace is necessary consider whether it should be a top-level
   namespace (e.g. `service`) or a nested namespace (e.g. `service.instance`).
 
-- Semantic conventions MUST limit attribute names to printable Basic Latin
-  characters (more precisely to
+- Semantic conventions MUST limit names to printable Basic Latin characters
+  (more precisely to
   [U+0021 .. U+007E](https://en.wikipedia.org/wiki/Basic_Latin_(Unicode_block)#Table_of_characters)
-  subset of Unicode code points). It is recommended to further limit attribute
-  names to the following Unicode code points: Latin alphabet, Numeric,
-  Underscore, Dot (as namespace delimiter).
+  subset of Unicode code points). It is recommended to further limit names to
+  the following Unicode code points: Latin alphabet, Numeric, Underscore, Dot
+  (as namespace delimiter).
 
-#### Recommendations for Application Developers
+### Recommendations for Application Developers
 
-As an application developer when you need to record an attribute first consult
-existing semantic conventions for
+As an application developer when you need to record an attribute or a label
+first consult existing semantic conventions for
 [Resources](https://github.com/open-telemetry/opentelemetry-specification/tree/master/specification/resource/semantic_conventions),
 [Spans](https://github.com/open-telemetry/opentelemetry-specification/tree/master/specification/trace/semantic_conventions),
 and
 [Metrics](https://github.com/open-telemetry/opentelemetry-specification/tree/master/specification/metrics/semantic_conventions).
-If appropriate attribute name does not exists you will need to come up with a
-new name. To do that consider a few options:
-
-- The attribute is specific to your company and may be possibly used outside the
-  company as well. To avoid name clashes with attributes introduced by other
-  companies (in a distributed system that uses applications from multiple
-  vendors) it is recommended to prefix the attribute name by your company's
-  reverse domain name, e.g. `com.acme.shopname`.
-
-- The attribute is specific to your application that will be used internally
-  only. If you already have an internal company process that helps you to ensure
-  no name clashes happen then feel free to follow it. Otherwise it is
-  recommended to prefix the attribute name by your application name, provided
-  that the application name is reasonably unique within your organization (e.g.
+If an appropriate name does not exists you will need to come up with a new name.
+To do that consider a few options:
+
+- The name is specific to your company and may be possibly used outside the
+  company as well. To avoid clashes with names introduced by other companies (in
+  a distributed system that uses applications from multiple vendors) it is
+  recommended to prefix the new name by your company's reverse domain name, e.g.
+  `com.acme.shopname`.
+
+- The name is specific to your application that will be used internally only. If
+  you already have an internal company process that helps you to ensure no name
+  clashes happen then feel free to follow it. Otherwise it is recommended to
+  prefix the attribute name or label key by your application name, provided that
+  the application name is reasonably unique within your organization (e.g.
   `myuniquemapapp.longitude` is likely fine). Make sure the application name
   does not clash with an existing semantic convention namespace.
 
-- The attribute may be generally applicable to applications in the industry. In
-  that case consider submitting a proposal to this specification to add a new
-  attribute to the semantic conventions, if necessary also to add a new
-  namespace for the attribute.
+- The name may be generally applicable to applications in the industry. In that
+  case consider submitting a proposal to this specification to add a new name to
+  the semantic conventions, and if necessary also to add a new namespace.
 
-It is recommended to limit attribute names to printable Basic Latin characters
+It is recommended to limit names to printable Basic Latin characters
 (more precisely to
 [U+0021 .. U+007E](https://en.wikipedia.org/wiki/Basic_Latin_(Unicode_block)#Table_of_characters)
 subset of Unicode code points).