Add external tag to opentrace events

[iilyak]

Overview

This PR adds an ability to selectively enable opentracing for HTTP requests
with X-B3-... headers. This is helpful in following cases:

• tracing all requests with X-B3-... headers
  all = (#{external := E}) when E == true -> true
• tracing all requests to specific database with X-B3-... headers

  all = (#{external := E, 'db.name' := Db})
    when E == true andalso Db == <<"foo">> -> true
  

• tracing requests to specific endpoint with X-B3-... headers

  db.design.view.read = (#{external := E, 'design.id' := Name})
    when E == true andalso Name == <<"bar">> -> true
  

I want to remind that we support following X-B3-... headers:

• X-B3-TraceId
• X-B3-SpanId
• X-B3-ParentSpanId
• B3 which is in the following format
  <TraceId>-<SpanId>-<1 | 0>-<ParentSpanId>

Testing recommendations

0. Run test suite

$ make eunit apps=ctrace

1. Write connection string into fdb.cluster file
2. Start FDB server
3. Update rel/overlay/etc/default.ini similar to this:

[tracing]
enabled = true
thrift_format = compact ; compact | binary
agent_host = 127.0.0.1
agent_port = 6831
app_name = couchdb

[tracing.filters]
; Log all traces
all = (#{external := External}) when External == true -> true

4. Start jaeger docker run -d -p 6831:6831/udp -p 16686:16686 jaegertracing/all-in-one:1.14
5. Start couchdb dev/run --admin=adm:pass
6. Create database curl -X PUT -u adm:pass http://127.0.0.1:15984/test
7. Query database curl -X GET -u adm:pass http://127.0.0.1:15984/test -H 'X-B3-TraceId: 12345678901234567890123453782027'
8. Open http://localhost:16686/ in the browser and search for spans
9. Query database curl -X GET -u adm:pass http://127.0.0.1:15984/test
10. Make sure there are no traces for the call from step #9

As an alternative to Step 4, you can run the jaeger-all-in-one binary without arguments in a separate shell. The Jaeger project provides prebuilt binaries for download.

Related Issues or Pull Requests

Checklist

• Code is written and works correctly
• Changes are covered by tests
• Any new configurable parameters are documented in rel/overlay/etc/default.ini
• A PR for documentation changes has been made in https://github.com/apache/couchdb-documentation

[nickva]

+1

Nicely done

I had a question about a warning in when examining the spans but I think that's because it's an external span?