Add the HTTP header format proposal for TraceContext propagation. (#1)

* Add the HTTP header format proposal for TraceContext propagation.

* Fix the first round of comments.

.gitignore

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+# IntelliJ IDEA
+.idea
+*.iml

HTTP_HEADER_FORMAT.md

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+# Trace Context HTTP Header Format
+
+A trace context header is used to pass trace context information across systems
+for a HTTP request. Our goal is to share this with the community so that various
+tracing and diagnostics products can operate together, and so that services can
+pass context through them, even if they're not being traced (useful for load
+balancers, etc.)
+
+# Format
+
+## Header name
+
+`Trace-Context`
+
+## Field value
+
+`base16(<version>)-<version_format>)`
+
+The value will be US-ASCII encoded (which is UTF-8 compliant). Character `-` is
+used as a delimiter between fields.
+
+### Version
+
+Is a 1-byte representing a 8-bit unsigned integer. Version 255 reserved.
+
+### Version = 0
+
+#### Format
+
+`base16(<trace-id>)-base16(<span-id>)-base16(<trace-options>)`
+
+All fields are required. Character `-` is used as a delimiter between fields.
+
+#### Trace-id
+
+Is the ID of the whole trace forest. It is represented as a 16-bytes array,
+e.g., `4bf92f3577b34da6a3ce929d0e0e4736`. All bytes 0 is considered invalid.
+
+Implementation may decide to completely ignore the trace-context if the trace-id
+is invalid.
+
+#### Span-id
+
+Is the ID of the caller span (parent). It is represented as a 8-bytes array,
+e.g., `00f067aa0ba902b7`. All bytes 0 is considered invalid.
+
+Implementation may decide to completely ignore the trace-context if the span-id
+is invalid.
+
+#### Trace-options
+
+Controls tracing options such as sampling, trace level etc. It is a 1-byte
+representing a 8-bit unsigned integer. The least significant bit provides
+recommendation whether the request should be traced or not (1 recommends the
+request should be traced, 0 means the caller does not make a decision to trace
+and the decision might be deferred). The flags are recommendations given by the
+caller rather than strict rules to follow for 3 reasons:
+
+1.  Trust and abuse.
+2.  Bug in caller
+3.  Different load between caller service and callee service might force callee
+    to down sample.
+
+The behavior of other bits is currently undefined.
+
+#### Examples of HTTP headers
+
+*Valid sampled Trace-Context:*
+
+```
+Value = 00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01
+base16(<Version>) = 00
+base16(<TraceId>) = 4bf92f3577b34da6a3ce929d0e0e4736
+base16(<SpanId>) = 00f067aa0ba902b7
+base16(<TraceOptions>) = 01  // sampled
+```
+
+*Valid not-sampled Trace-Context:*
+
+```
+Value = 00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-00
+base16(<Version>) = 00
+base16(<TraceId>) = 4bf92f3577b34da6a3ce929d0e0e4736
+base16(<SpanId>) = 00f067aa0ba902b7
+base16(<TraceOptions>) = 00  // not-sampled
+```