Runtime environment metrics namespace guideline is outdated

[joaopgrassi]

In our current runtime environment metrics guidelines we say:

Metrics specific to a certain runtime environment should be prefixed with process.runtime.{environment}

But in #227, JVM metrics were moved to to jvm.* rendering the guidelines there outdated.

We need to probably either remove the guidelines or adapt it to say the metrics should be under the runtime's top-level namespace.