Improve metric markdown generation

[joaopgrassi]

Support to generate markdown for metrics from yaml was added in open-telemetry/build-tools#79.

I'm currently moving the existing "hard-coded" metric semantic convention from the markdown files to yaml and I noticed a few things that can be improved:

• Generate metric name/header/description automatically:

For example, in the runtime-environment-metrics the metrics are organized as such:

### Metric: `process.runtime.jvm.memory.usage` (1)

This metric is [recommended][MetricRecommended]. (2)
...

When moving the metrics to YAML, to be able to achieve the same output as above, I need to manually add the (1) header and the recommendation.

1. The header we can generate, since we have all the information
2. The "This metric is recommended.." I don't think we have this field in the data model, so we might need to include it

In the end, it would be great if the only thing required to generate consistent markdown, are the placeholders such as

<!-- semconv metric.system.cpu.time(metric_table) -->
<!-- endsemconv -->

<!-- semconv metric.system.cpu.time(full) -->
<!-- endsemconv -->

And that would generate

### Metric: `process.runtime.jvm.memory.usage`

This metric is [recommended][MetricRecommended].

<metric table>

<metric attribute table>

[joaopgrassi]

We have also the status which has a field in YAML that we can use to render in markdown, instead of "hard-coding" it. In some cases the document has mixed statuses like the http metrics semconv and it would be nice if that can also be auto-generated from the YAML definitions.

[Oberon00]

Note that the same / something very similar applies to other types of attributes: #1086

[lmolkova]

We're adding stability column in the open-telemetry/build-tools#278.

Also adding related issues to this bigger one

• Render requirement level column on metrics build-tools#279
• Add histogram bucket boundaries to metric semconv yaml files #1225
• Improve markdown table generation for metrics build-tools#129

[jsuereth]

@joaopgrassi I'd love to see your request moved over into semconv repo, as I think you could implement it after #1000 is merged.

[jsuereth]

@joaopgrassi I think you should be able to expand support for metric tables directly in semconv via the JINJA templates now.