Include support for a new <springProfile> element which can be used in
`logback-spring.xml` files to selectively enable or disable parts of the
configuration. For example:

	<configuration>
		...
		<springProfile name="staging">
			<logger name="sample.logback" level="TRACE" />
		</springProfile>
		...
	</configuration>

Fixes gh-3338

Add a logback sample application to demonstrate how `logback-spring.xml`
can be used for customization.

See gh-2558

Update AbstractLoggingSystem to support convention based logging
configuration specifically for Spring applications. Configurations with
the `-spring` suffix will now be loaded automatically (for example
`logback-spring.xml`).

This change allows for custom log configurations without needing to
reinitialize the logging system. When standard configurations are used
there is no way to prevent early initialization.

Fixes gh-2558

The default is now to reveal all details unless sensitive=true
(instead of only revealing then if sensitive was explicitly false).
The definition of "secure" also changes to something more sensible
where it is only true if security is enabled.

Fixes gh-2816

The default should be to generate a key from the prefix. Otherwise
if user sets the prefix and not the key it can pick up metrics from
another repository (which is what happens in the tests).

* pr/3335:
  Remove unnecessary import
  Remove unnecessary `@After` callback

See gh-3335

`@After` callback was trying to remove the wrong file anyway. This is
now taken care of by the `TemporaryFolder` rule.

Closes gh-3335

* pr/3333:
  Fix typo

Closes gh-3333

Update LogbackLoggingSystem to include status errors in the exception
rather than using `System.err`. Also perform additional cleanup in an
attempt to fix CI build failures.

Fixes gh-3309

Fixes build break introduced in 31336c2d.

Closes gh-3309

Closes gh-3330

Improve the "hints" section of the metadata so that each hint can provide
the reference to a value provider.

A value provider defines how a tool can discover the potential values of
a property based on the context. The provider is identifed by a name and
may have an arbitrary number of parameters.

Closes gh-3303

* patch-25:
  Fix typos

Closes gh-3326

* pr/3314:
  Use `bytes.length` in Base64Encoder loop

Update Base64Encoder to use `bytes.length` instead of `encoded.length`
when lopping over the bytes.

Fixes gh-3314

* pr/3320:
  Fix type in deployment documentation

Replace use of `link -s` with `ln -s`.

Fixes gh-3320

* pr/3321:
  Update documentation to remove 'shaded jar' term

Update documentation section that discusses uber jars since it conflates
the concepts of the uber jar and the shading of dependencies into jars
(which may or may not be true uber jars).

Fixes gh-3321

Change FlywayMigrationStrategy from a class to an interface.

Fixes gh-3217

Extract common features from CounterBuffers and GuageBuffers into
a shared superclass. The new extracted types allows the service
implementations to be simplified.

Fixes gh-3257

Update EventPublishingRunListener to implement Ordered and use a default
order of 0. This allows for other run listeners to be added either
before or after it.

Fixes gh-3305

Create a new section in the meta-data called "hints" where users can
provide hints about a given property. The most basic use case for now
is to provide a list of values that a property can have. Each value may
have a description.

This sample JSON provides a basic example for a property called `foo.mode`
that exposes 3 values: "auto", "basic" and "advanced".

```
 "hints": [
    {
      "id": "foo.mode",
      "values": [
        {
          "value": "auto",
          "description": "Some smart description."
        },
        {
          "name": "basic"
        },
        {
          "name": "advanced"
        }
      ]
    }
]
```

This information can be read by tools (such as IDE) and offer an
auto-completion with the list of values.

Closes gh-2054

If java.io.tmpdir is configured to a directory that does not exist,
calls to File.createTempFile will fail with an IOException with the
message "The system cannot find the path specified". Unfortunately,
the path the was specified is not included in the message.

Rather than trying to automatically create the directory in what may
be a misconfigured location, we now include the value of
java.io.tmpdir in our own exception's message. Hopefully this will
help users to figure out what they've done wrong.

Closes gh-3307

The maximum and minimum number of connections are no metrics so these
are not exposed. Fix the doc that stated the contrary.

Fixes gh-3319