Functions for collecting context from various sources and merging into a
collected context map.

Some example sources of context data:
- environmental data about stack/sys/env/host names
- request call graph context from prodigal
- tracing span information

Core user API namespace for ken instrumentation.

There are really three significant phases where data is collected to form an
event or 'span':
1. At the _beginning_ of a span, to initialize the event data:
   - base: label, time, level
   - callsite: ns, line
   - trace: trace-id, parent-id, span-id
   - sampling: sample-rate, keep?
2. _During_ a span:
   - using annotate, error, and time
3. At the _end_ of a span:
   - context collection
   - thread name
   - duration

An `observe` call performs the first and third phases back-to-back and
immediately sends an event, while `watch` will wrap a body of code, providing
time to enrich it with extra data in phase two.

Base event attributes and functions.

Observability instrumentation using a global 'tap' which can have events
reported to it from any location in the code. This draws inspiration from
Clojure's `tap>`, but is more directly targeted at instrumentation.

Tracing support for instrumenting code.

Trace data is structured as events with a few special properties:
- `::trace/trace-id`
  The parent identifier for the overall trace of all linked events.
- `::trace/span-id`
  A unique identifier for this event.
- `::trace/parent-id`
  The span which is the parent of this one.
- `::trace/keep?`
  A boolean to represent a sampling decision for events sent to ken on whether to keep or drop the spans of a trace.
  - The absence of this key means the observed event will be kept and forwarded along. This is the default state.
  - `false` means to discard a span. This decision will be propagated to child spans, but can be overridden later by a child span that decides to force keep.
  - `true` means to forcibly keep a span and all of its child spans, ignoring all other sampling decisions in the subtree.

Additionally, trace events must have:
- `::event/time`
  This indicates the instant the span started.
- `::event/duration`
  The duration (in milliseconds) the span covers.