Loki: Change live tailing to only allow mutating the log line not the number of streams.

[slim-bean]

What this PR does / why we need it:

Stops live tailing from splitting a stream into more streams when using parsers while still maintaining the ability to mutate the log line using parsers and line_format

The current implementation will allow creating many new streams from a tailed stream when using parsers, but this multiplicative affect can make it difficult to then tail the stream because it creates a lot more overhead in the amount of data pushed down the websocket.

For the purposes of live tailing which is meant primarily for human consumption, returning the parsed and extracted labels and manipulating the streams isn't helpful for the end user visualization and instead makes the process less efficient and more likely to struggle to keep up with higher volume streams.

Update: Also included in this PR is a change to no longer reuse the same pipeline for parsing every push request, the pipeline has a label cache built into it which, when reused, would lead to unbounded memory growth when the log lines being processed had extremely high cardinality data being parsed.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Documentation added
• Tests updated
• Is this an important fix or new feature? Add an entry in the CHANGELOG.md.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/upgrading/_index.md

[owen-d]

Ooh, nice find. How'd you catch this?

[sandeepsukhani]

Nice one! LGTM!