Minor misconfiguration causes errors to be thrown

[NickStallman]

Description

It's possible to have a misconfiguration that throws some errors visibly on the website.

Warning: Unknown: [ddappsec] Failed to add tag _dd.appsec.event_rules.version in Unknown on line 0
Warning: Unknown: [ddappsec] Expecting an object from \ddtrace\root_span in Unknown on line 0
Warning: Unknown: [ddappsec] Expecting an object from \ddtrace\root_span in Unknown on line 0
Warning: Unknown: [ddappsec] Failed to add _dd.runtime_family to root span in Unknown on line 0
Warning: Unknown: [ddappsec] Expecting an object from \ddtrace\root_span in Unknown on line 0

You get this when you have:

datadog.trace.enabled = Off
datadog.appsec.enabled = On

It's a silly configuration, however I'd expect it to throw a warning in the PHP log file when the module is loaded, and reset appsec.enabled to Off if it doesn't detect trace.enabled is On.
I wouldn't expect that kind of error to be thrown on page loads.

Tested on the latest versions of ddtrace and appsec.

[edsrzf]

I'm seeing this behavior with what I believe is a reasonable configuration.

I have:

• DD_TRACE_ENABLED=0 in the environment, which should be equivalent to datadog.trace.enabled = Off
• Defaults in the configuration. According to the documentation, the default is supposed to be datadog.appsec.enabled = Off, but this is not exactly true.

The default for datadog.appsec.enabled depends on datadog.remote_config_enabled, which defaults to On. I've created a separate issue for this default in #242.