ddtrace/tracer: implement fmt.Formatter interface to support log injection

[knusbaum]

This change implements fmt.Formatter to allow printing a span in various formats for logging.

[jdgumz]

LGTM on last comment for tests.

[gbbr]

I think we need to do our best to find the information needed in a log, even if the tracer might be stopped. If the tracer is absent, we can still get a:

• Service by checking globalconfig.ServiceName()
• Env by checking DD_ENV
• Version by checking DD_VERSION

We shouldn't give up as easily by leaving them empty.

[knusbaum]

Hmmm. I'm not sure how best to implement picking up env, version, and service from WithGlobalTag and DD_TAGS.
WithGlobalTag options get called after defaults() are set and env vars are picked up, but should only set version, env, and service in certain cases.

i.e.
with DD_TAGS == version:1.2.3, and WithGlobalTag("version", "2.3.4"), WithGlobalTag should take precedence over DD_TAGS and version should be 2.3.4

But
With DD_VERSION == 1.2.3, and WithGlobalTag("version", "2.3.4"), WithGlobalTag should not take precedence, and version should be 1.2.3.

Currently there's no way for WithGlobalTag to know whether version was set from DD_TAGS or from DD_VERSION.

[gbbr]

Hmmm. I'm not sure how best to implement picking up env, version, and service from WithGlobalTag and DD_TAGS.

If you mean specifically in the fallback when no tracer is started, in that case ignore these. It's just a fallback. 99.9% of the time this won't happen. Plus, tags are not the right place to set service, env and version.

[knusbaum]

Sorry for causing confusion here. I have deleted some of my comments around DD_TAGS to reduce noise.

I will put up another PR around picking up env, version, and service from DD_TAGS and WithGlobalTag and we can discuss it there.

This PR handles only the logging of env, version, and service.