Allow specifying a tag on NpgsqlCommand that gets reported in tracing

[roji]

To help view and process tracing data, we could allow users to set an arbitrary string tag on NpgsqlCommand, which would get reported when tracing via OpenTelemetry.

Originally raised in https://github.com/dotnet/efcore/issues/33149#issuecomment-1962348729

[MrLesk]

Hi @roji we are trying to integrate Datadog pg_tracing in our services.

Since we are using EF, we can use interceptors to add the trace info as a comment ahead of each query. This works for us but I was wondering if we could have some support in npgsql so that we could do the same at a lower level. For example we are also using Masstransit with npgsql and there we don't have an interceptor option.

[vonzshik]

At the lowest level, theoretically, you can already do that by using Activity.Current.

using var cmd = new NpgsqlCommand(...);
using var reader = cmd.ExecuteReader();
Activity.Current?.SetTag("SomeKey", "SomeValue");
// Handle reader

[MrLesk]

Yeah but I would like to do it from configuration as I don’t always have access to npgsql directly.

something like otel.addnpgsql(options=> { options.addW3cTracingContext = true; }