expfmt: throw error when metric's name not provided instead panicing

[Alhanaqtah]

This PR enhances extfmt behavior when incorrect metrics privided by user — metrics without name. Now TextParser throws error "invalid metrics name" instead of panicing (#3374)

@roidelapluie

[Alhanaqtah]

@roidelapluie could you review PR, please?)

[ywwg]

This change looks good to me. Can you add a test to exercise this case so we can catch regressions? And can you sign your commit to pass the DCO check? Thanks!

[ywwg]

Hi, I would like to do a release of prom/common and this would be a nice fix to include, do you think you can add the test and fix DCO soon?

[Alhanaqtah]

Hi, I would like to do a release of prom/common and this would be a nice fix to include, do you think you can add the test and fix DCO soon?

I've done everything. You can merge it)

P.S.
I also extended (*TextParser).reset() with p.currentMetric assignment to nil so that testcases wouldn't affect each other