Warn when choosing invalid metric names

[spladug]

If a span is named something that doesn't make a valid metric path component, it can cause the metrics related to it to get discard. For example, a redis pipeline named foo:bar would cause a metric like clients.redis.pipeline_foo:bar which is invalid. This should not fail silently.

[rishabhRsinghvi]

@spladug is it still left to be done? 😂