Clarify unit annotation format for metric semantic conventions

[arminru]

The semantic conventions for metrics currently use both spaces and underscores in unit annotations consisting of multiple words. Additionally, we have examples where the spaces between words are just omitted.

Before calling any of the metrics semantic conventions stable, we should add guidance on what's preferred and align the existing conventions to be consistent.

Also, as per the UCUM standard, both 1 and {count} are considered equivalent, which might lead to inconsistent conventions. A preference for either of them could also be laid out by the OTel spec.

Examples

opentelemetry-specification/specification/metrics/semantic_conventions/instrumentation/kafka.md

Lines 40 to 42 in 534d456

	| messaging.kafka.partition.current_offset | Gauge | Int64 | partition offset | `{partition offset}` | Current offset of partition of topic. | `topic` | The ID (integer) of a topic |
	| | | | | | | `partition` | The number (integer) of the partition |
	| messaging.kafka.partition.oldest_offset | Gauge | Int64 | partition offset | `{partition offset}` | Oldest offset of partition of topic | `topic` | The ID (integer) of a topic |

opentelemetry-specification/specification/metrics/semantic_conventions/instrumentation/kafka.md

Lines 78 to 83 in 534d456

	| messaging.kafka.consumer.offset_sum | Gauge | Int64 | offset sum | `{offset sum}` | Sum of consumer group offset across partitions of topic | `group` | The ID (string) of a consumer group |
	| | | | | | | `topic` | The ID (integer) of a topic |
	| messaging.kafka.consumer.lag | Gauge | Int64 | lag | `{lag}` | Current approximate lag of consumer group at partition of topic | `group` | The ID (string) of a consumer group |
	| | | | | | | `topic` | The ID (integer) of a topic |
	| | | | | | | `partition` | The number (integer) of the partition |
	| messaging.kafka.consumer.lag_sum | Gauge | Int64 | lag sum | `{lag sum}` | Current approximate sum of consumer group lag across all partitions of topic | `group` | The ID (string) of a consumer group |

opentelemetry-specification/specification/metrics/semantic_conventions/instrumentation/kafka.md

Line 65 in 534d456

| messaging.kafka.producer.record-sent.rate | Gauge | Double | records sent rate | `{records_sent}/s` | The average number of records sent per second for a specific topic. | `client-id` | `client-id` value |

opentelemetry-specification/specification/metrics/semantic_conventions/faas-metrics.md

Line 43 in 534d456

| `faas.coldstarts` | Counter | default unit | `{coldstarts}` | Number of invocation cold starts. |

[tigrannajaryan]

@arminru do yo plan to work on this? If not we need a "help wanted" label.

[evan-bradley]

I'd like to take a look at this. @jsuereth do you see this issue depending on any output from the Semantic Conventions Working Group, or can it be worked on in parallel?