Prom-client high memory usage

[shappir]

I have a service that uses several histograms, some with high cardinality. As a result, the retained memory size just for those histograms is 25-30MB. I'm hoping that this commit, and this PR will significantly improve the situation.

[shappir]

Another cause for the large memory consumption are the long hash key values used: all the label name / value pairs concatenated into a string. Here are possible ways to reduce its size:

• Only use the values, not the names. Since the pairs are sorted by name, that will be unique (label names don't add uniqueness)
• Instead of using the actual string as hash key, compute shorter unique hash from it. For example, using a function like this one: https://stackoverflow.com/questions/7616461/generate-a-hash-from-string-in-javascript/52171480#52171480 (It's Open Source: https://github.com/mbaersch/cyrb53-hasher ) Computing this hash looks cheap, and the lookup might actually be faster.