SeverityNumber=0 MAY be used to represent an unspecified value (#4535)

Fixes
#4478

## Changes

Per
#4509 (comment)

> > The same is probably done in many languages. Shouldn't we say
somewhere in specification that 0 is a "reserved" value as it may be
used to model unspecified?
> 
> Yes, I think we can.

The main purpose of this PR would then be to allow to have `0` as a
"reserved" value as this is how most implementations implement the
"unspecified" value e.g.
-
[OTLP](https://github.com/open-telemetry/opentelemetry-proto/blob/68f9c6329ca91f96333ee5b240e305945b122b70/opentelemetry/proto/logs/v1/logs.proto#L88)
- [OTel
Collector](https://github.com/open-telemetry/opentelemetry-collector/blob/e1f670844604a5b119d8560bc079ceca4c92bf72/pdata/plog/severity_number.go#L14)
- [OTel
Java](https://github.com/open-telemetry/opentelemetry-java/blob/a756317511741fa06ad343c12d3599c1b7618a4d/api/all/src/main/java/io/opentelemetry/api/logs/Severity.java#L14)
- [OTel
Go](https://github.com/open-telemetry/opentelemetry-go/blob/1636bcdd1d59fd7284ee2c65ee22acd038d1e979/log/severity.go#L17)
- [OTel
PHP](https://github.com/open-telemetry/opentelemetry-php/blob/65c29795232fe96664bd95f417bb2c6e879b5489/src/API/Logs/LogRecord.php#L16)
- [OTel
Python](https://github.com/open-telemetry/opentelemetry-python/blob/12bcd4508e4aca43667450db090a50b9efab9963/opentelemetry-api/src/opentelemetry/_logs/severity/__init__.py#L31)
- [OTel
Ruby](https://github.com/open-telemetry/opentelemetry-ruby/blob/4621c02e5b9e476a4fcd892173c55f91500aa699/logs_api/lib/opentelemetry/logs/severity_number.rb#L10)
- [OTel
C++](https://github.com/open-telemetry/opentelemetry-cpp/blob/d2e728914d290e9e3dc7a97cba848cc055f53653/api/include/opentelemetry/logs/severity.h#L24)
- [OTel
JS](https://github.com/open-telemetry/opentelemetry-js/blob/main/experimental/packages/api-logs/src/types/LogRecord.ts#L24)

Defining `0` as something else would be disruptive for these languages.
If it would be safe to "reserve" this value and make it optional.

I guess only [OTel
Rust](https://github.com/open-telemetry/opentelemetry-rust/blob/7b3db0b6a692574ff4d504ad2b6cd5634316e3a0/opentelemetry/src/logs/record.rs#L141-L192)
and [OTel
Swift](https://github.com/open-telemetry/opentelemetry-swift/blob/a235bc4636fe0df030fb8a0e9193e0d0367f2238/Sources/OpenTelemetryApi/Logs/Severity.swift#L8-L33)
have "proper" enums which does not require handling `0` as unspecified.

Author: pellared

CHANGELOG.md

@@ -27,6 +27,8 @@ release.
   ([#4534](https://github.com/open-telemetry/opentelemetry-specification/pull/4534))
 - Stabilize SDK and No-Op `Logger.Enabled`.
   ([#4536](https://github.com/open-telemetry/opentelemetry-specification/pull/4536))
+- `SeverityNumber=0` MAY be used to represent an unspecified value.
+  ([#4535](https://github.com/open-telemetry/opentelemetry-specification/pull/4535))
 
 ### Baggage
 

specification/logs/data-model.md

@@ -303,6 +303,8 @@ events. Larger numerical values in each range represent more important (more
 severe) events. For example `SeverityNumber=17` describes an error that is less
 critical than an error with `SeverityNumber=20`.
 
+`SeverityNumber=0` MAY be used to represent an unspecified value.
+
 #### Mapping of `SeverityNumber`
 
 Mappings from existing logging systems and formats (or **source format** for