Define Meters

[PascalSenn]

We need to define which OTEL Meters we find relevant for GraphQL.

Please comment on this issue with ideas.

Examples:
graphql.server.active_operations -> up/down counter
graphql.server.parsing.duration -> histogram
graphql.server.variable_coercion.duration -> histogram
graphql.server.validation.duration -> histogram

[benjie]

A few more ideas:

• Errors histogram
• Cached parse/validate vs fresh document
• APQ success vs uncached
• Response size (though this is more HTTP in general I guess)