Add semantic conventions for log media attributes (#2022)

Logs are transmitted via several common media, such as files and
iostreams. This PR establishes semantic conventions for describing the
file or iostream from which a log was read. Notably, these attributes
are not resource attributes, as the medium used to tranmit a log is not
considered to be the originator of the log.

specification/logs/semantic_conventions/README.md

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+# Log Attribute Semantic Conventions
+
+**Status**: [Experimental](../../document-status.md)
+
+The following semantic conventions for logs are defined:
+
+* [Log Media](media.md): Semantic attributes that may be used in describing the source of a log.
+
+Apart from semantic conventions for logs, [traces](../../trace/semantic_conventions/README.md), and [metrics](../../metrics/semantic_conventions/README.md),
+OpenTelemetry also defines the concept of overarching [Resources](../../resource/sdk.md) with their own
+[Resource Semantic Conventions](../../resource/semantic_conventions/README.md).

specification/logs/semantic_conventions/media.md

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+# Semantic Conventions for Log Media
+
+**Status**: [Experimental](../../document-status.md)
+
+This document describes attributes for log media in OpenTelemetry. Log media are mechanisms by which logs are transmitted. Types of media include files, streams, network protocols, and os-specific logging services such as journald and Windows Event Log.
+
+<!-- Re-generate TOC with `markdown-toc --no-first-h1 -i` -->
+
+<!-- toc -->
+
+- [Log Media](#log-media)
+  * [Log File](#log-file)
+  * [I/O Stream](#io-stream)
+
+<!-- tocstop -->
+
+## Log Media
+
+**Note:** The OpenTelemetry specification defines a [Resource](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/resource/sdk.md#resource-sdk) as `an immutable representation of the entity producing telemetry`.
+The following attributes do not describe entities that produce telemetry. Rather, they describe mechanisms of log transmission.
+As such, these should be recorded as Log Record attributes when applicable. They should not be recorded as Resource attributes.
+
+### Log File
+
+**Description:** A file to which log was emitted.
+
+| Name                            | Notes and examples                                                                       |
+| ------------------------------- | ---------------------------------------------------------------------------------------- |
+| `log.file.name`          | The basename of the file. Example: `audit.log`                                           |
+| `log.file.path`          | The full path to the file. Example: `/var/log/mysql/audit.log`                           |
+| `log.file.name_resolved` | The basename of the file, with symlinks resolved. Example: `uuid.log`                    |
+| `log.file.path_resolved` | The full path to the file, with symlinks resolved. Example: `/var/lib/docker/uuid.log`   |
+
+### I/O Stream
+
+**Description:** The I/O stream to which the log was emitted.
+
+| Name                            | Notes and examples                                                                       |
+| ------------------------------- | ---------------------------------------------------------------------------------------- |
+| `log.iostream`        | The stream associated with the log. SHOULD be one of: `stdout`, `stderr` |