Require that names and namespaces are one global space across all sem…

…antic convention areas

We have semantic conventions for Resources, Spans and Metrics (in the future also Logs are expected).
It was not clear if the attribute names across all convention areas should be globally unique.

This commit asserts that conventions are one space, they are not independent spaces with
their own namespaces each.

We prohibit using the same Span or Resource attribute name or metric label name but give them
slightly different meanings or value sets.

Resolves: #815

specification/common/common.md

@@ -90,6 +90,13 @@ Names SHOULD follow these rules:
 - 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 exist for 4 areas: for Resource, Span and Log attribute
+  and for Metric label keys. Identical namespaces or names in these areas MUST
+  have identical meanings. For example the `http.method` span attribute name
+  denotes exactly the same concept as the `http.method` metric label, has the
+  same data type and the same set of possible values (in both cases it records
+  the value of the HTTP protocol's request method as a string).
+
 - 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)