Add Logs to OpenTelemetry vocabulary (open-telemetry/oteps#91)

* Add Logs to OpenTelemetry vocabulary

This commit proposes the initial vocabulary for logs and event to be used across
OpenTelemetry project.

This is an initial proposal and we can extend / clarify the vocabulary as we go.

* Address PR comments

oteps/0091-logs-vocabulary.md

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+# Logs: Vocabulary
+
+This documents defines the vocabulary for logs to be used across OpenTelemetry project.
+
+## Motivation
+
+We need a common language and common understanding of terms that we use to
+avoid the chaos experienced by the builders of the Tower of Babel.
+
+## Proposal
+
+OpenTelemetry specification already contains a [vocabulary](https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/overview.md)
+for Traces, Metrics and other relevant concepts.
+
+This proposal is to add the following concepts to the vocabulary.
+
+### Log Record
+
+A recording of an event. Typically the record includes a timestamp indicating
+when the event happened as well as other data that describes what happened,
+where it happened, etc.
+
+Also known as Log Entry.
+
+### Log
+
+Sometimes used to refer to a collection of Log Records. May be ambiguous, since
+people also sometimes use `Log` to refer to a single `Log Record`, thus this
+term should be used carefully and in the context where ambiguity is possible
+additional qualifiers should be used (e.g. `Log Record`).
+
+### Embedded Log
+
+`Log Records` embedded inside a [Span](https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/api-tracing.md#span) 
+object, in the [Events](https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/api-tracing.md#add-events) list.
+
+### Standalone Log
+
+`Log Records` that are not embedded inside a `Span` and are recorded elsewhere.
+
+### Log Attributes
+
+Key/value pairs contained in a `Log Record`.
+
+### Structured Logs
+
+Logs that are recorded in a format which has a well-defined structure that allows
+to differentiate between different elements of a Log Record (e.g. the Timestamp,
+the Attributes, etc). The _Syslog protocol_ ([RFC 5425](https://tools.ietf.org/html/rfc5424)),
+for example, defines a `structured-data` format.
+
+### Flat File Logs
+
+Logs recorded in text files, often one line per log record (although multiline
+records are possible too). There is no common industry agreement whether
+logs written to text files in more structured formats (e.g. JSON files)
+are considered Flat File Logs or not. Where such distinction is important it is
+recommended to call it out specifically.