Add CHANGELOG.md (#18)

CHANGELOG.md

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+# Changelog
+
+Please update changelog as part of any significant pull request. Place short
+description of your change into "Unreleased" section. As part of release process
+content of "Unreleased" section content will generate release notes for the
+release.
+
+## Unreleased
+
+### Semantic Conventions
+
+- Clarify the scope of the HTTP client span.
+ ([#3290](https://github.com/open-telemetry/opentelemetry-specification/pull/3290))
+- Add moratorium on relying on schema transformations for telemetry stability
+  ([#3380](https://github.com/open-telemetry/opentelemetry-specification/pull/3380))
+- Mark "Instrumentation Units" and "Instrumentation Types" sections of the general
+  metric semantic conventions as stable
+  ([#3294](https://github.com/open-telemetry/opentelemetry-specification/pull/3294))
+- Add clarification that UCUM c/s variant applies to all units other than `1` and
+  those using [annotations](https://ucum.org/ucum.html#para-curly).
+  ([#3393](https://github.com/open-telemetry/opentelemetry-specification/pull/3393))
+- Specify that seconds should be used for measuring durations.
+  ([#3388](https://github.com/open-telemetry/opentelemetry-specification/pull/3388))
+- Change http.server.duration and http.client.duration units to seconds
+  ([#3390](https://github.com/open-telemetry/opentelemetry-specification/pull/3390))
+- BREAKING: Remove `messaging.consumer.id`, make `messaging.client_id` generic
+  ([#3336](https://github.com/open-telemetry/opentelemetry-specification/pull/3336))
+- Add transition plan for upcoming breaking changes to the unstable HTTP semantic
+  conventions.
+  ([#3443](https://github.com/open-telemetry/opentelemetry-specification/pull/3443))
+- Rename `net.peer.*`, `net.host.*`, and `net.sock.*` attributes to align with ECS
+  ([#3402](https://github.com/open-telemetry/opentelemetry-specification/pull/3402))
+    BREAKING: rename `net.peer.name` to `server.address` on client side and to `client.address` on server side,
+     `net.peer.port` to `server.port` on client side and to `client.port` on server side,
+     `net.host.name` and `net.host.port` to `server.name` and `server.port` (since `net.host.*` attributes only applied to server instrumentation)
+     `net.sock.peer.addr` to `server.socket.address` on client side and to `client.socket.address` on server side,
+     `net.sock.peer.port` to `server.socket.port` on client side and to `client.socket.port` on server side,
+     `net.sock.peer.name` to `server.socket.domain` (since `net.sock.peer.name` only applied to client instrumentation),
+     `net.sock.host.addr` to `server.socket.address` (since `net.sock.host.*` only applied to server instrumentation),
+     `net.sock.host.port` to `server.socket.port` (similarly since `net.sock.host.*` only applied to server instrumentation),
+     `http.client_ip` to `client.address`
+- BREAKING: Introduce `network.transport` defined as
+  [OSI Transport Layer](https://osi-model.com/transport-layer/) or
+  [Inter-process Communication method](https://en.wikipedia.org/wiki/Inter-process_communication).
+  Introduce `network.type` defined as [OSI Network Layer](https://osi-model.com/network-layer/)
+  or non-OSI equivalent. Remove `net.transport` and `net.sock.family`.
+  Rename `net.protocol.*` to `network.protocol.*`,
+  `net.host.connection.*` to `network.connection.*`, and
+  `net.host.carrier.*` to `network.carrier.*`.
+  ([#3426](https://github.com/open-telemetry/opentelemetry-specification/pull/3426))
+- BREAKING: Adopt ECS attributes in HTTP semantic conventions.
+  Renames: `http.method` to `http.request.method`,
+  `http.status_code` to `http.response.status_code`,
+  `http.request_content_length` to `http.request.body.size`,
+  `http.response_content_length` to `http.response.body.size`,
+  `http.url` to `url.full`,
+  `http.scheme` to `url.scheme`,
+  and removes `http.target` breaking it down to `http.target` to `url.path`, `url.query`, and `url.fragment`.
+  ([#3355](https://github.com/open-telemetry/opentelemetry-specification/pull/3355))
+- Add `gcp.cloud_run.job.execution` and `gcp.cloud_run.job.task_id` resource
+  attributes for GCP Cloud Run Jobs ([#3378](https://github.com/open-telemetry/opentelemetry-specification/pull/3378))