Support use of `TagWith` in EntityFramework

[stevejgordon]

In EF, the TagWith API lets users tag queries with information that is then included in the generated SQL statement.

Our SignatureParser.Parse method doesn't support tags, so they are trimmed as '-- in the span name.

We can review the work required to trim tags from the statement before we attempt to match the first "real" token with the SQL operation.

[oteinone]

Would you be open to contributions for this issue? My company would definitely get more out of Elastic Apm if we could get better span descriptions from Entity Framework queries. We have 90% of "--" and "SELECT" in our APM dependencies instead of actual table names. I was able to resolve most of these anonymous sql queries with (at least superficially) quite minor changes. Initial benchmarks did not show major performance hits.

I changed comment handling (do not end token scan to comments) and select handling (keep searching for table name if the table name is in an inner query). Obviously this is just a first attempt but I could spend a few days on this if I have a chance to get something merged :)

My changeset: https://github.com/oteinone/apm-agent-dotnet/tree/feature_enhancements