Edit pass on Semantic Conventions for HTTP Metric

Edit pass on [Semantic Conventions for HTTP Metrics](https://github.com/open-telemetry/semantic-conventions/blob/main/docs/http/http-metrics.md#semantic-conventions-for-http-metrics)

See [this PR](https://github.com/open-telemetry/semantic-conventions/pull/415/files)

Make [Attribute Requirement Levels for Semantic Conventions](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/common/attribute-requirement-level.md#attribute-requirement-levels-for-semantic-conventions) attributes code styled

do a global search and replace (where appropriate) of
`, otherwise ` to `; otherwise, `

For example, MUST be used; otherwise, a custom value MAY be used.