Add agent semantic conventions

Resolves open-telemetry#396
Contributes to open-telemetry/opamp-spec#131

We need a way to record more information about agents than is currently
possible using existing semantic conventions. Otel Collector in particular
today uses service.name,service.instance.id,service.version attributes
to report its own telemetry. These are useful but not sufficient, particularly
we are missing the information about which distribution of Otel Collector
it is.

agent.type/agent.version/agent.id conventions are also aligned with ECS:
https://www.elastic.co/guide/en/ecs/current/ecs-agent.html

With introduction of this conventions the following attributes change in
Otel Collector's own telemetry output:

service.name -> agent.type
service.version -> agent.version
service.instance.id -> agent.id

agent.distro will be added as one more property, the equivalent of which
did not exist in the past.

docs/attributes-registry/agent.md

@@ -0,0 +1,21 @@
+<!--- Hugo front matter used to generate the website version of this page:
+--->
+
+# Agent
+
+## Agent Attributes
+
+<!-- semconv registry.agent(omit_requirement_level) -->
+| Attribute  | Type | Description  | Examples  | Stability |
+|---|---|---|---|---|
+| `agent.distro` | string | Agent distribution. [1] | `github.com/signalfx/splunk-otel-collector`; `github.com/aws-observability/aws-otel-collector` | ![Experimental](https://img.shields.io/badge/-experimental-blue) |
+| `agent.id` | string | Unique identifier of agent instance. [2] | `627cc493-f310-47de-96bd-71410b7dec09` | ![Experimental](https://img.shields.io/badge/-experimental-blue) |
+| `agent.type` | string | Agent type. [3] | `io.opentelemetry.collector`; `com.dynatrace.one_agent`; `com.newrelic.infra_agent` | ![Experimental](https://img.shields.io/badge/-experimental-blue) |
+| `agent.version` | string | The version string of the agent. The format is not defined by these conventions. | `2.0.0`; `a01dbef8a` | ![Experimental](https://img.shields.io/badge/-experimental-blue) |
+
+**[1]:** Identifies the distribution of the agent. A number of distributions can belong to the same `agent.type`. For example OpenTelemetry Collector has multiple known distributions, e.g. github.com/signalfx/splunk-otel-collector, github.com/aws-observability/aws-otel-collector, etc.  The value is typically a URL where the agent's source code is hosted  (without the preceding http/https scheme). However other approaches for choosing `agent.distro` values are also valid (e.g. reverse FQDN).
+
+**[2]:** If a deterministic source for an id is not available it is recommended to use a UUID v7 value.
+
+**[3]:** A string that uniquely identifies the agent type. A recommended way to choose a value is to pick a reverse FQDN of a domain that is under the control of the agent's author. Must remain unchanged between different versions of the same agent type.
+<!-- endsemconv -->

docs/resource/agent.md

@@ -0,0 +1,18 @@
+<!--- Hugo front matter used to generate the website version of this page:
+--->
+
+# Agent
+
+## Agent Attributes
+
+<!-- semconv agent(full) -->
+| Attribute  | Type | Description  | Examples  | [Requirement Level](https://opentelemetry.io/docs/specs/semconv/general/attribute-requirement-level/) | Stability |
+|---|---|---|---|---|---|
+| [`agent.type`](../attributes-registry/agent.md) | string | Agent type. [1] | `io.opentelemetry.collector`; `com.dynatrace.one_agent`; `com.newrelic.infra_agent` | `Required` | ![Experimental](https://img.shields.io/badge/-experimental-blue) |
+| [`agent.distro`](../attributes-registry/agent.md) | string | Agent distribution. [2] | `github.com/signalfx/splunk-otel-collector`; `github.com/aws-observability/aws-otel-collector` | `Recommended` | ![Experimental](https://img.shields.io/badge/-experimental-blue) |
+| [`agent.version`](../attributes-registry/agent.md) | string | The version string of the agent. The format is not defined by these conventions. | `2.0.0`; `a01dbef8a` | `Recommended` | ![Experimental](https://img.shields.io/badge/-experimental-blue) |
+
+**[1]:** A string that uniquely identifies the agent type. A recommended way to choose a value is to pick a reverse FQDN of a domain that is under the control of the agent's author. Must remain unchanged between different versions of the same agent type.
+
+**[2]:** Identifies the distribution of the agent. A number of distributions can belong to the same `agent.type`. For example OpenTelemetry Collector has multiple known distributions, e.g. github.com/signalfx/splunk-otel-collector, github.com/aws-observability/aws-otel-collector, etc.  The value is typically a URL where the agent's source code is hosted  (without the preceding http/https scheme). However other approaches for choosing `agent.distro` values are also valid (e.g. reverse FQDN).
+<!-- endsemconv -->

model/registry/agent.yaml

@@ -0,0 +1,51 @@
+groups:
+  - id: registry.agent
+    prefix: agent
+    type: attribute_group
+    brief: >
+        An agent.
+    attributes:
+      - id: type
+        type: string
+        stability: experimental
+        brief: >
+          Agent type.
+        examples: [io.opentelemetry.collector,com.dynatrace.one_agent,com.newrelic.infra_agent]
+        note: >
+          A string that uniquely identifies the agent type. A recommended way to choose
+          a value is to pick a reverse FQDN of a domain that is under the control of the
+          agent's author. Must remain unchanged between different versions of the same
+          agent type.
+
+      - id: distro
+        type: string
+        stability: experimental
+        brief: >
+          Agent distribution.
+        examples: [github.com/signalfx/splunk-otel-collector, github.com/aws-observability/aws-otel-collector]
+        note: >
+          Identifies the distribution of the agent. A number of
+          distributions can belong to the same `agent.type`. For example OpenTelemetry
+          Collector has multiple known distributions, e.g.
+          github.com/signalfx/splunk-otel-collector,
+          github.com/aws-observability/aws-otel-collector, etc. 
+          The value is typically a URL where the agent's source code is hosted 
+          (without the preceding http/https scheme). However other approaches for choosing
+          `agent.distro` values are also valid (e.g. reverse FQDN).
+
+      - id: version
+        type: string
+        stability: experimental
+        brief: >
+          The version string of the agent. The format is not defined by these conventions.
+        examples: ["2.0.0", "a01dbef8a"]
+
+      - id: id
+        type: string
+        stability: experimental
+        brief: >
+          Unique identifier of agent instance.
+        examples: ["627cc493-f310-47de-96bd-71410b7dec09"]
+        note: >
+          If a deterministic source for an id is not available it is recommended to use
+          a UUID v7 value.

model/resource/agent.yaml

@@ -0,0 +1,11 @@
+groups:
+  - id: agent
+    prefix: agent
+    type: resource
+    brief: >
+        An agent
+    attributes:
+      - ref: agent.type
+        requirement_level: required
+      - ref: agent.version
+      - ref: agent.distro