Add conventions for attribute names (open-telemetry#807)

* Add conventions for attribute names

Resolves: open-telemetry/opentelemetry-specification#726

* Address PR comments

* Re-word company/application specific attribute recommendations

specification/common/common.md

@@ -6,7 +6,7 @@ Table of Contents
 </summary>
 
 - [Attributes](#attributes)
-
+  - [Attribute Naming](#attribute-naming)
 </details>
 
 ## Attributes
@@ -36,3 +36,89 @@ This is required for map/dictionary structures represented as two arrays with
 indices that are kept in sync (e.g., two attributes `header_keys` and `header_values`,
 both containing an array of strings to represent a mapping
 `header_keys[i] -> header_values[i]`).
+
+### Attribute Naming
+
+Attribute name (also known as the "attribute key") MUST be a valid Unicode
+sequence.
+
+Attribute 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
+  `service` is the namespace and `version` is an attribute in that namespace.
+
+- Namespaces can be nested. For example `telemetry.sdk` is a namespace inside
+  top-level `telemetry` namespace and `telemetry.sdk.name` is an attribute
+  inside `telemetry.sdk` namespace.
+
+- For each multi-word dot-delimited component of the attribute name separate the
+  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
+  `service.instance.id` is an attribute name then it is no longer valid to have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.
+
+#### Recommendations for OpenTelemetry Authors
+
+- All attribute 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
+  [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.
+
+- 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
+  [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).
+
+#### Recommendations for Application Developers
+
+As an application developer when you need to record an attribute 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.
+  `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.
+
+It is recommended to limit attribute 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).
+