Add fallback for host.id in Cloud

If the instance_id can not be retrieved, use machine-id value.

docs/attributes-registry/host.md

@@ -14,7 +14,7 @@
 | `host.cpu.model.name` | string | Model designation of the processor. | `11th Gen Intel(R) Core(TM) i7-1185G7 @ 3.00GHz` |
 | `host.cpu.stepping` | int | Stepping or core revisions. | `1` |
 | `host.cpu.vendor.id` | string | Processor manufacturer identifier. A maximum 12-character string. [1] | `GenuineIntel` |
-| `host.id` | string | Unique host ID. For Cloud, this must be the instance_id assigned by the cloud provider. For non-containerized systems, this should be the `machine-id`. See the table below for the sources to use to determine the `machine-id` based on operating system. | `fdbf79e8af94cb7f9e8df36789187052` |
+| `host.id` | string | Unique host ID. For Cloud, this must be the instance_id assigned by the cloud provider. If the instance_id can not be retrieved, this should be the `machine-id`. For non-containerized systems, this should be the `machine-id`. See the table below for the sources to use to determine the `machine-id` based on operating system. | `fdbf79e8af94cb7f9e8df36789187052` |
 | `host.image.id` | string | VM image ID or host OS image ID. For Cloud, this value is from the provider. | `ami-07b06b442921831e5` |
 | `host.image.name` | string | Name of the VM image or OS install the host was instantiated from. | `infra-ami-eks-worker-node-7d4ec78312`; `CentOS-8-x86_64-1905` |
 | `host.image.version` | string | The version string of the VM image or host OS as defined in [Version Attributes](/docs/resource/README.md#version-attributes). | `0.1` |

docs/resource/host.md

@@ -13,7 +13,7 @@ To report host metrics, the `system.*` namespace SHOULD be used.
 | Attribute  | Type | Description  | Examples  | Requirement Level |
 |---|---|---|---|---|
 | [`host.arch`](../attributes-registry/host.md) | string | The CPU architecture the host system is running on. | `amd64` | Recommended |
-| [`host.id`](../attributes-registry/host.md) | string | Unique host ID. For Cloud, this must be the instance_id assigned by the cloud provider. For non-containerized systems, this should be the `machine-id`. See the table below for the sources to use to determine the `machine-id` based on operating system. | `fdbf79e8af94cb7f9e8df36789187052` | Recommended |
+| [`host.id`](../attributes-registry/host.md) | string | Unique host ID. For Cloud, this must be the instance_id assigned by the cloud provider. If the instance_id can not be retrieved, this should be the `machine-id`. For non-containerized systems, this should be the `machine-id`. See the table below for the sources to use to determine the `machine-id` based on operating system. | `fdbf79e8af94cb7f9e8df36789187052` | Recommended |
 | [`host.image.id`](../attributes-registry/host.md) | string | VM image ID or host OS image ID. For Cloud, this value is from the provider. | `ami-07b06b442921831e5` | Recommended |
 | [`host.image.name`](../attributes-registry/host.md) | string | Name of the VM image or OS install the host was instantiated from. | `infra-ami-eks-worker-node-7d4ec78312`; `CentOS-8-x86_64-1905` | Recommended |
 | [`host.image.version`](../attributes-registry/host.md) | string | The version string of the VM image or host OS as defined in [Version Attributes](/docs/resource/README.md#version-attributes). | `0.1` | Recommended |