Enhancement: Add duration type parameter

[agardnerIT]

Related to #51

Tracepusher currently accepts -dur N where it is hardcoded to be dealt with as seconds. tracepusher then translates that to nano seconds: https://github.com/agardnerIT/tracepusher/blob/cfec80193bd0d4a2770989a32f941c53670af406/tracepusher.py#L222

This issue is created for an enhancement to tracepusher that:

• Introduces a new (optional) parameter which allows users to specify a duration type (eg. milliseconds, seconds etc.)
• Supported values will be ms and s
• As discussed in #opentelemetry CNCF slack, traces are best when (wall-clock) short. So tracepusher shouldn't disusade this by offering spans of anything more than seconds. In other words, a --duration-type of minutes min or hours h is not appropriate.
• If not specified, tracepusher should be backwards compatible: ie. should default to the current behaviour which is a duration type of seconds.

[agardnerIT]

Work in progress: https://github.com/agardnerIT/tracepusher/blob/add-duration-type/tracepusher.py

[agardnerIT]

Resolved: https://github.com/agardnerIT/tracepusher/commit/ebd8af49dfbe38a98c05fefffc203801b5a78949

Will add docs now